Method and device for encoding or decoding image on basis of inter mode

ABSTRACT

In a method and a device for encoding or decoding an image according to the present invention, motion information for bidirectional prediction of a current block may be derived on the basis of an inter mode previously defined in the device for encoding or decoding an image, and inter prediction may be performed on the current block on the basis of the motion information, wherein the motion information for bidirectional prediction is adjusted to be motion information for unidirectional prediction according to the predefined inter mode.

TECHNICAL FIELD

The present disclosure relates to a video encoding/decoding method andapparatus.

BACKGROUND ART

As a demand for high-resolution and high-definition video has recentlyincreased, a need for a high-efficiency video compression technology fornext-generation video services has emerged. Based on this need, ISO/IECMPEG and ITU-T VCEG, which jointly standardized H.264/AVC and HEVC videocompression standards, formed JVET (Joint Video Exploration Team) andconducted research and exploration to establish a new video compressionstandard from October 2015. In April 2018, a new video compressionstandardization was started with an evaluation of a responses to a newvideo compression standard CfP (Call for Proposal).

In a video compression technique, a block division structure means aunit that performs encoding and decoding, and a unit to which majorencoding and decoding techniques such as prediction and transformationare applied. As video compression technology develops, the size ofblocks for encoding and decoding is gradually increasing, and morevarious division types are supported as a block division type. Inaddition, video compression is performed using not only units forencoding and decoding, but also units subdivided according to the roleof blocks.

In the HEVC standard, video encoding and decoding are performed using aunit block subdivided according to a quad-tree type block divisionstructure and a role for prediction and transformation. In addition tothe quad-tree type block division structure, various types of blockdivision structures such as QTBT (Quad Tree plus Binary Tree) in theform of combining a quad-tree and a binary-tree, and MTT (Multi-TypeTree) in which a triple-tree is combined therewith have been proposed toimprove video coding efficiency. Through the support of various blocksizes and various types of block division structures, one picture isdivided into multiple blocks, and information in units of coding unitssuch as a coding mode, motion information, and intra predictiondirection information corresponding to each block is expressed invarious ways, so the number of bits expressing this is increasingsignificantly.

DISCLOSURE Technical Problem

An image encoding/decoding method and apparatus according to the presentdisclosure provides a limited bidirectional prediction method.

An image encoding/decoding method and apparatus according to the presentdisclosure defines various inter modes for improving an efficiency ofinter prediction, and provides a method of inducing motion informationaccording to each inter mode.

An image encoding/decoding method and apparatus according to the presentdisclosure provides a method of adaptively limiting consumption of amemory bandwidth by using a plurality of motion vectors.

An image encoding/decoding method and apparatus according to the presentdisclosure provides a hierarchical division method of a picture.

Technical Solution

An image encoding/decoding method and apparatus according to the presentdisclosure may derive motion information for bidirectional prediction ofa current block, based on an inter mode pre-defined in an image decodingapparatus, and perform inter prediction on the current block based onthe motion information.

In the image encoding/decoding method and apparatus according to thepresent disclosure, according to the pre-defined inter mode, the motioninformation for the bidirectional prediction may be adjusted to motioninformation for unidirectional prediction.

In the image encoding/decoding method and apparatus according to thepresent disclosure, when the pre-defined inter mode is a merge mode, themotion information for the bidirectional prediction may be adjusted tomotion information for unidirectional prediction, and when thepre-defined inter mode is an affine mode, the motion information for thebidirectional prediction may be not adjusted to motion information forunidirectional prediction.

In the image encoding/decoding method and apparatus according to thepresent disclosure, when the pre-defined inter mode is a merge mode, themotion information for the bidirectional prediction may be adjusted tomotion information for unidirectional prediction in consideration of asize of the current block.

In the image encoding/decoding method and apparatus according to thepresent disclosure, when the pre-defined inter mode is a merge mode, thederiving of the motion information may comprise constructing a mergecandidate list of the current block, and deriving motion information ofthe current block from the merge candidate list.

In the image encoding/decoding method and apparatus according to thepresent disclosure, the merge candidate list may include at least one ofa spatial merge candidate, a temporal merge candidate, or a combinedmerge candidate.

In the image encoding/decoding method and apparatus according to thepresent disclosure, when the pre-defined inter mode is an affine mode,the deriving of the motion information may comprise generating acandidate list consisting of affine candidates of the current block,deriving a control point vector of the current block based on thecandidate list and a candidate index, and deriving a motion vector ofthe current block based on the control point vector of the currentblock.

In the image encoding/decoding method and apparatus according to thepresent disclosure, the affine candidates may include at least one of aspatial candidate, a temporal candidate, or a configured candidate.

In the image encoding/decoding method and apparatus according to thepresent disclosure, in consideration of a size of a predeterminedreference region, a motion vector of the current block is derived, andthe reference region includes a reference block specified by motionvectors of n sub-blocks and a pixel used to interpolate a fractional pelof the reference block.

In the image encoding/decoding method and apparatus according to thepresent disclosure, when the size of the reference region is less thanor equal to a threshold value, the motion vector of the current blockmay be derived for each sub-block, and when the size of the referenceregion is greater than the threshold value, one motion vector may bederived for the current block.

In the image encoding/decoding method and apparatus according to thepresent disclosure, the motion vector of the current block may bederived based on an affine motion parameter, and the affine motionparameter may be calculated based on the control point vector of thecurrent block.

In the image encoding/decoding method and apparatus according to thepresent disclosure, the current block may belong to a predeterminedsub-picture, and the sub-picture may be defined as a group of gridshaving the same index among a plurality of grids constituting onepicture.

Advantageous Effects

The present disclose may allow bidirectional prediction only when apredetermined condition is satisfied, or may reduce the consumption of amemory bandwidth by adjusting a motion vector.

According to the present disclosure, an efficiency of inter predictionmay be improved by using a combined merge candidate in addition tospatial/temporal merge candidates.

According to the present disclosure, an efficiency of imageencoding/decoding may be improved through inter prediction based on anaffine motion model.

According to the present disclosure, an efficiency of imageencoding/decoding may be improved by dividing one picture intomulti-level fragment regions.

DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an image encoding apparatus accordingto the present disclosure.

FIG. 2 is a block diagram showing an image decoding apparatus accordingto the present disclosure.

FIG. 3 illustrates a limited bidirectional prediction process as anembodiment to which the present disclosure is applied.

FIG. 4 illustrates a concept of performing inter prediction using affinetransform prediction in a video coding method and apparatus according toan embodiment of the present disclosure.

FIG. 5 illustrates a concept of affine transform prediction in units ofsub-blocks according to an embodiment of the present disclosure.

FIG. 6 illustrates an affine mode based inter prediction method as anembodiment to which the present disclosure is applied.

FIG. 7 illustrates a concept of limiting a motion vector inconsideration of a memory bandwidth as an embodiment to which thepresent disclosure is applied.

FIG. 8 illustrates an inter prediction method using mask-basedprediction in a video coding method and apparatus according to anembodiment of the present disclosure.

FIG. 9 illustrates a concept of bidirectional prediction in a videoencoding and/or decoding method and apparatus according to an embodimentof the present disclosure.

FIG. 10 illustrates a concept of limited bidirectional prediction as anembodiment to which the present disclosure is applied.

FIG. 11 is a flowchart of a process of a decoding predictiondirectionality according to an embodiment of the present disclosure.

FIG. 12 is a flowchart for determining whether a predictiondirectionality is limited according to an embodiment of the presentdisclosure.

FIG. 13 is a flowchart for determining whether a predictiondirectionality is limited according to another embodiment of the presentdisclosure.

FIG. 14 illustrates a concept of dividing one picture using a unitdefined as multiple layers according to an embodiment of the presentdisclosure.

FIG. 15 illustrates a concept of dividing an image, an image group, or avideo into a plurality of tiles and processing them into three tilegroups according to an embodiment of the present disclosure.

FIG. 16 illustrates a concept in which an image, an image group, or animage is divided into a plurality of tiles, and only a reconstructedimage corresponding to each tile is shared and restored according to anembodiment of the present disclosure.

FIG. 17 illustrates an example in which an image, an image group, or avideo are divided into a plurality of tiles, and each tile is dividedinto sub-tiles according to an embodiment of the present disclosure.

BEST MODE FOR INVENTION

An image encoding/decoding method and apparatus according to the presentdisclosure may derive motion information for bidirectional prediction ofa current block, based on an inter mode pre-defined in an image decodingapparatus, and perform inter prediction on the current block based onthe motion information.

In the image encoding/decoding method and apparatus according to thepresent disclosure, according to the pre-defined inter mode, the motioninformation for the bidirectional prediction may be adjusted to motioninformation for unidirectional prediction.

In the image encoding/decoding method and apparatus according to thepresent disclosure, when the pre-defined inter mode is a merge mode, themotion information for the bidirectional prediction may be adjusted tomotion information for unidirectional prediction, and when thepre-defined inter mode is an affine mode, the motion information for thebidirectional prediction may be not adjusted to motion information forunidirectional prediction.

In the image encoding/decoding method and apparatus according to thepresent disclosure, when the pre-defined inter mode is a merge mode, themotion information for the bidirectional prediction may be adjusted tomotion information for unidirectional prediction in consideration of asize of the current block.

In the image encoding/decoding method and apparatus according to thepresent disclosure, when the pre-defined inter mode is a merge mode, thederiving of the motion information may comprise constructing a mergecandidate list of the current block, and deriving motion information ofthe current block from the merge candidate list.

In the image encoding/decoding method and apparatus according to thepresent disclosure, the merge candidate list may include at least one ofa spatial merge candidate, a temporal merge candidate, or a combinedmerge candidate.

In the image encoding/decoding method and apparatus according to thepresent disclosure, when the pre-defined inter mode is an affine mode,the deriving of the motion information may comprise generating acandidate list consisting of affine candidates of the current block,deriving a control point vector of the current block based on thecandidate list and a candidate index, and deriving a motion vector ofthe current block based on the control point vector of the currentblock.

In the image encoding/decoding method and apparatus according to thepresent disclosure, the affine candidates may include at least one of aspatial candidate, a temporal candidate, or a configured candidate.

In the image encoding/decoding method and apparatus according to thepresent disclosure, in consideration of a size of a predeterminedreference region, a motion vector of the current block is derived, andthe reference region includes a reference block specified by motionvectors of n sub-blocks and a pixel used to interpolate a fractional pelof the reference block.

In the image encoding/decoding method and apparatus according to thepresent disclosure, when the size of the reference region is less thanor equal to a threshold value, the motion vector of the current blockmay be derived for each sub-block, and when the size of the referenceregion is greater than the threshold value, one motion vector may bederived for the current block.

In the image encoding/decoding method and apparatus according to thepresent disclosure, the motion vector of the current block may bederived based on an affine motion parameter, and the affine motionparameter may be calculated based on the control point vector of thecurrent block.

In the image encoding/decoding method and apparatus according to thepresent disclosure, the current block may belong to a predeterminedsub-picture, and the sub-picture may be defined as a group of gridshaving the same index among a plurality of grids constituting onepicture.

Mode for Invention

Hereinafter, embodiments of the present disclosure will be described indetail with reference to the accompanying drawings in the presentspecification so that those of ordinary skill in the art may easilyimplement the present disclosure. However, the present disclosure may beimplemented in various different forms and is not limited to theembodiments described herein. In the drawings, parts irrelevant to thedescription are omitted in order to clearly describe the presentdisclosure, and similar reference numerals are attached to similar partsthroughout the specification.

Throughout this specification, when a certain part is said to be‘connected’ with another part, this includes not only the case where itis directly connected, but also the case where it is electricallyconnected with another element in the middle. In addition, in the entirespecification, when a certain part “includes” a certain component, itmeans that other components may be further included rather thanexcluding other components unless otherwise stated.

The terms ‘step (to)˜’ or ‘step of˜’ as used throughout thisspecification does not mean ‘step for˜’. In addition, terms such asfirst and second may be used to describe various elements, but theelements should not be limited to the terms. The above terms are usedonly for the purpose of distinguishing one component from anothercomponent.

In addition, the components shown in the embodiment of the presentdisclosure are shown independently to represent different characteristicfunctions, it does not mean that each component is made of separatehardware or a single software component unit. That is, each componentunit is described by being listed as a respective component unit forconvenience of description, and at least two of the component units arecombined to form one component unit, or one component unit may bedivided into a plurality of component units to perform a function. Anintegrated embodiment and a separate embodiment of each of thesecomponents are also included in the scope of the present disclosure aslong as they do not depart from the essence of the present disclosure.

In the various embodiments of the present disclosure described hereinbelow, terms such as “˜unit”, “˜group”, “˜unit”, “˜module”, and “˜block”mean units that process at least one function or operation, and they maybe implemented in hardware or software, or a combination of hardware andsoftware.

In addition, a coding block refers to a processing unit of a set oftarget pixels on which encoding and decoding are currently performed,and may be used interchangeably as a coding block and a coding unit. Inaddition, the coding unit refers to a coding unit (CU) and may begenerically referred to including a coding block (CB).

In addition, quad-tree division refers to that one block is divided intofour independent coding units, and binary division refers to that oneblock is divided into two independent coding units. In addition, ternarydivision refers to that one block is divided into three independentcoding units in a 1:2:1 ratio.

FIG. 1 is a block diagram showing an image encoding apparatus accordingto the present disclosure.

Referring to FIG. 1 , a video encoding apparatus 100 may include: apicture dividing module 110, prediction modules 120 and 125, a transformmodule 130, a quantization module 135, a rearrangement module 160, anentropy encoding module 165, an inverse quantization module 140, aninverse transform module 145, a filter module 150, and a memory 155.

A picture dividing module 110 may divide an input picture into one ormore processing units. Herein, the processing unit may be a predictionunit (PU), a transform unit (TU), or a coding unit (CU). Hereinafter, inan embodiment of the present disclosure, a coding unit may be used as aunit that performs encoding or a unit that performs decoding.

A prediction unit may be resulting from dividing one coding unit into atleast one square or non-square of the same size, and it may be dividedsuch that one prediction unit among prediction units divided within onecoding unit has a different shape and/or size from another predictionunit. When it is not a minimum coding unit in generating a predictionunit which performs intra prediction based on a coding unit, intraprediction may be performed without dividing the coding unit into aplurality of prediction units N×N.

Prediction modules 120 and 125 may include an inter prediction module120 performing inter prediction and an intra prediction module 125performing intra prediction. Whether to perform inter prediction orintra prediction for a prediction unit may be determined, and detailedinformation (e.g., an intra prediction mode, a motion vector, areference picture, etc.) according to each prediction method may bedetermined. A residual value (residual block) between a generatedprediction block and an original block may be input to a transformmodule 130. In addition, prediction mode information, motion vectorinformation, etc. used for prediction may be encoded together with aresidual value by an entropy encoding module 165 and may be transmittedto a decoder. However, when a motion information derivation techniquefrom the side of a decoder according to the present disclosure isapplied, since an encoder does not generate prediction mode informationand motion vector information, the corresponding information is nottransmitted to the decoder. On the other hand, it is possible for anencoder to signal and transmit information indicating that motioninformation is derived and used from the side of a decoder andinformation on a technique used for inducing the motion information.

A inter prediction module 120 may predict a prediction unit based oninformation of at least one of a previous picture or a subsequentpicture of a current picture, or may predict a prediction unit based oninformation of some encoded regions in the current picture, in somecases. The inter prediction module 120 may include a reference pictureinterpolation module, a motion prediction module, and a motioncompensation module.

A reference picture interpolation module may receive reference pictureinformation from a memory 155 and may generate pixel information on aninteger pixel or less than the integer pixel from the reference picture.In the case of luma pixels, an 8-tap DCT-based interpolation filterhaving different filter coefficients may be used to generate pixelinformation on an integer pixel or less than the integer pixel in a unitof a ¼ pixel. In the case of chroma signals, a 4-tap DCT-basedinterpolation filter having different filter coefficients may be used togenerate pixel information on an integer pixel or less than the integerpixel in a unit of a ⅛ pixel.

A motion prediction module may perform motion prediction based on areference picture interpolated by a reference picture interpolationmodule. As a method for obtaining a motion vector, various methods suchas a full search-based block matching algorithm (FBMA), a three stepsearch (TSS), and a new three-step search algorithm (NTS) may be used. Amotion vector may have a motion vector value in a unit of ½ pixel or a ¼pixel based on an interpolated pixel. A motion prediction module maypredict a current prediction unit by using various motion predictionmethods. As a motion prediction method, various methods such as a skipmethod, a merge method, an Advanced Motion Vector Prediction (AMVP)method, and an intra block copy method may be used. In addition, whenapplying a motion information derivation technique from the side of adecoder according to the present disclosure, as a method performed by amotion prediction module, a template matching method and a bilateralmatching method using a motion trajectory may be applied. In connection,the template matching method and the bilateral matching method will bedescribed in detail later in FIG. 3 .

An intra prediction module 125 may generate a prediction unit based onreference pixel information neighboring to a current block which ispixel information in the current picture. When a neighboring block of acurrent prediction unit is a block on which inter prediction has beenperformed and a reference pixel is a pixel on which inter prediction hasbeen performed, a reference pixel included in a block on which interprediction has been performed may be replaced with reference pixelinformation of a neighboring block on which intra prediction has beenperformed. In other words, when a reference pixel is not available,information on a reference pixel that is not available may be replacedwith at least one reference pixel among available reference pixels.

In addition, a residual block including residual information that is adifference between a prediction unit on which prediction has beenperformed based on the prediction unit generated by prediction modules120 and 125 and an original block of the prediction unit may begenerated. The generated residual block may be input to a transformmodule 130.

A transform module 130 may transform a residual block including residualinformation between an original block and a prediction unit generated byprediction modules 120 and 125 using a transform method such as discretecosine transform (DCT), discrete sine transform (DST), and KLT. Whetherto apply DCT, DST, or KLT in order to transform a residual block may bedetermined based on intra prediction mode information of a predictionunit used to generate a residual block.

A quantization module 135 may quantize values transformed to a frequencydomain by a transform module 130. Quantization coefficients may varydepending on a block or importance of a picture. The values calculatedby a quantization module 135 may be provided to an inverse quantizationmodule 140 and a rearrangement module 160.

A rearrangement module 160 may rearrange coefficient values on quantizedresidual values.

A rearrangement module 160 may change coefficients in the form of atwo-dimensional block into coefficients in the form of a one-dimensionalvector through a coefficient scanning method. For example, arearrangement module 160 may scan from DC coefficients to coefficientsin a high frequency domain using zig-zag scanning method so as to changethe coefficients to be in the form of a one-dimensional vector.Depending on a size of a transform unit and an intra prediction mode,vertical scanning where coefficients in the form of a two-dimensionalblock are scanned in a column direction or horizontal scanning wherecoefficients in the form of a two-dimensional block are scanned in a rowdirection may be used instead of zig-zag scanning. In other words, whichscanning method among zig-zag scanning, vertical scanning, andhorizontal scanning is used may be determined depending on a size of atransform unit and an intra prediction mode.

An entropy encoding module 165 may perform entropy encoding based onvalues calculated by a rearrangement module 160. Entropy encoding mayuse various encoding methods such as Exponential Golomb,Context-Adaptive Variable Length Coding (CAVLC), and Context-AdaptiveBinary Arithmetic Coding (CABAC). In relation to this, an entropyencoding module 165 may encode residual value coefficient information ofa coding unit from a rearrangement module 160 and prediction modules 120and 125. In addition, according to the present disclosure, informationindicating that motion information is derived and used at a decoder sideand information on a technique used to derive motion information may besignaled and transmitted.

An inverse quantization module 140 and an inverse transform module 145may inversely quantize values quantized by a quantization module 135 andinversely transform values transformed by a transform module 130. Aresidual value generated by an inverse quantization module 140 and aninverse transform module 145 may be combined with a prediction unitpredicted through a motion prediction module, motion compensationmodule, and intra prediction module included in prediction modules 120and 125 to generate a reconstructed block.

A filter module 150 may include at least one of a deblocking filter, anoffset correction module, or an adaptive loop filter (ALF). A deblockingfilter may remove block distortion that occurs due to boundaries betweenblocks in a reconstructed picture. An offset correction module maycorrect offset with respect to an original image in a unit of a pixel ina deblocking filtered image. In order to perform offset correction on aparticular picture, a method of applying offset in consideration of edgeinformation of each pixel or a method of partitioning pixels included inan image into the predetermined number of regions, determining a regionto be subjected to perform offset, and applying the offset to thedetermined region may be used. Adaptive loop filtering (ALF) may beperformed based on a value obtained by comparing a filteredreconstructed image and an original image. After partitioning pixelsincluded in an image into predetermined groups, one filter to be appliedto the corresponding group may be determined, and filtering may beperformed differentially for each group.

A memory 155 may store a reconstructed block or picture calculatedthrough a filter module 150. The stored reconstructed block or picturemay be provided to prediction modules 120 and 125 in performing interprediction.

FIG. 2 is a block diagram showing an image decoding apparatus accordingto the present disclosure.

Referring to FIG. 2 , an apparatus 200 for decoding a video may include:an entropy decoding module 210, a rearrangement module 215, an inversequantization module 220, an inverse transform module 225, predictionmodules 230 and 235, a filter module 240, and a memory 245.

When a video bitstream is input to an apparatus for decoding a video,the input bitstream may be decoded according to an inverse process of anapparatus for encoding a video.

An entropy decoding module 210 may perform entropy decoding according toan inverse process of entropy encoding by an entropy encoding module ofa video encoding apparatus. For example, corresponding to methodsperformed by a video encoding apparatus, various methods such asExponential Golomb, Context-Adaptive Variable Length Coding (CAVLC), andContext-Adaptive Binary Arithmetic Coding (CABAC) may be applied.

An entropy decoding module 210 may decode information on intraprediction and inter prediction performed by an encoding apparatus.

A rearrangement module 215 may perform rearrangement on a bitstreamentropy decoded by an entropy decoding module 210 based on arearrangement method used in an encoding apparatus. A rearrangementmodule may reconstruct and rearrange coefficients in the form of aone-dimensional vector to coefficients in the form of a two-dimensionalblock.

An inverse quantization module 220 may perform inverse quantizationbased on a quantization parameter received from an encoding apparatusand rearranged coefficients of a block.

An inverse transform module 225 may perform inverse transform, i.e.,inverse DCT, inverse DST, and inverse KLT, which corresponds to atransform, i.e., DCT, DST, and KLT, performed by a transform module, ona quantization result by an apparatus for encoding a video. Inversetransform may be performed based on a transmission unit determined by avideo encoding apparatus. In an inverse transform module 225 of a videodecoding apparatus, transform schemes (e.g., DCT, DST, and KLT) may beselectively performed depending on multiple pieces of information suchas a prediction method, a size of a current block, and a predictiondirection.

Prediction modules 230 and 235 may generate a prediction block based oninformation on prediction block generation received from an entropydecoding module 210 and information on a previously decoded block orpicture received from a memory 245.

As described above, if a size of a prediction unit and a size of atransform unit are the same when intra prediction is performed in thesame manner as an operation of a video encoding apparatus, intraprediction may be performed on a prediction unit based on pixelsexisting on the left, upper left, and top of a prediction unit. However,if the size of the prediction unit and the size of the transform unitare different when the intra prediction is performed, intra predictionmay be performed using a reference pixel based on a transform unit. Inaddition, intra prediction using N×N division may be used only for theminimum coding unit.

Prediction modules 230 and 235 may include a prediction unitdetermination module, an inter prediction module, and an intraprediction module. A prediction unit determination module may receive avariety of information, such as prediction unit information, predictionmode information of an intra prediction method, and information onmotion prediction of an inter prediction method, from an entropydecoding module 210, may divide a current coding unit into predictionunits, and may determine whether inter prediction or intra prediction isperformed on the prediction unit. On the other hand, if an encoder 100does not transmit information related to motion prediction for interprediction, but transmit information indicating that motion informationis derived and used from the side of a decoder and information about atechnique used for deriving motion information, the prediction unitdetermination module determines prediction performance of an interprediction module 230 based on the information transmitted from theencoder 100.

An inter prediction module 230 may perform inter prediction on a currentprediction unit based on information of at least one of a previouspicture or a subsequent picture of the current picture including thecurrent prediction unit using information required for inter predictionof the current prediction unit provided by a video encoding apparatus.In order to perform inter prediction, based on a coding block, it may bedetermined whether a motion prediction method of a prediction unitincluded in a corresponding coding unit is a skip mode, a merge mode, anAMVP mode, or an intra block copy mode. Alternatively, the interprediction module 230 may itself derive motion information frominformation indicating that motion information is derived and used fromthe side of a decoder and information about a technique used to inducemotion information provided by a video encoder, and then perform interprediction.

An intra prediction module 235 may generate a prediction block based onpixel information in a current picture. When a prediction unit is aprediction unit subjected to intra prediction, intra prediction may beperformed based on intra prediction mode information of the predictionunit received from a video encoding apparatus. An intra predictionmodule 235 may include an adaptive intra smoothing (AIS) filter, areference pixel interpolation module, and a DC filter. An AIS filterperforms filtering on a reference pixel of a current block, and whetherto apply the filter may be determined depending on a prediction mode ofa current prediction unit. AIS filtering may be performed on a referencepixel of a current block by using a prediction mode of a prediction unitand MS filter information received from an apparatus for encoding avideo. When a prediction mode of a current block is a mode where AISfiltering is not performed, an AIS filter may not be applied.

When a prediction mode of a prediction unit is a prediction unit thatperforms intra prediction based on a pixel value interpolated by areference pixel, a reference pixel interpolation module may interpolatea reference pixel to generate a reference pixel in a unit of pixel equalto an integer pixel or less than the integer pixel. When a predictionmode of a current prediction unit is a prediction mode in which aprediction block is generated without interpolating a reference pixel, areference pixel may not be interpolated. A DC filter may generate aprediction block through filtering when a prediction mode of a currentblock is a DC mode.

A reconstructed block or picture may be provided to a filter module 240.A filter module 240 may include a deblocking filter, an offsetcorrection module, and an ALF.

Information on whether a deblocking filter is applied to a correspondingblock or picture and information on which of a strong filter and a weakfilter is applied when the deblocking filter is applied may be receivedfrom a video encoding apparatus. A deblocking filter of a video decodingapparatus may receive information on a deblocking filter from a videoencoding apparatus, and may perform deblocking filtering on acorresponding block.

An offset correction module may perform offset correction on areconstructed image based on a type of offset correction and offsetvalue information applied to an image in performing encoding. An ALF maybe applied to a coding unit based on information on whether to apply theALF, ALF coefficient information, etc. received from an encodingapparatus. The ALF information may be provided as being included in aparticular parameter set.

A memory 245 may store a reconstructed picture or block for use as areference picture or block, and may provide a reconstructed picture toan output module.

FIG. 3 illustrates a limited bidirectional prediction process as anembodiment to which the present disclosure is applied.

Referring to FIG. 3 , motion information of a current block may bederived based on an inter mode pre-defined in an encoding/decodingapparatus (S300).

A pre-defined inter mode according to the present disclosure may includeat least one of a merge mode, an AMVP mode, an affine mode, or an intrablock copy mode. Herein, motion information may be variously interpretedas a motion vector prediction value, a motion vector, a control pointvector prediction value, a control point vector, a block vector, and thelike according to an inter mode.

1. Merge mode

In the case of a merge mode, motion information of a current block maybe set to be the same as motion information of a merge candidate. Inthis way, motion information is derived through merging with a mergecandidate, and a separate motion vector difference value (mvd) is notsignaled. Hereinafter, a method of inducing motion information based ona merge mode will be described in detail.

First, a merge candidate list of a current block may be configured (S1).A merge candidate list may include at least one of a spatial mergecandidate or a temporal merge candidate of a current block.

Motion information of a spatial merge candidate may be derived frommotion information of a spatial neighboring block of a current block.Herein, the spatial neighboring block may be a block belonging to thesame picture as the current block, and may mean a block adjacent to thecurrent block. A spatial neighboring block may include a block adjacentto at least one of a left, top, top right, bottom left, or top left of acurrent block. A top left neighboring block may be used only when atleast one of blocks adjacent to a left, top, top right and bottom leftof a current block is not available.

Motion information of a temporal merge candidate may be derived frommotion information of a temporal neighboring block of a current block.The temporal neighboring block may be a block belonging to a picturedifferent from the current block and may be defined as a block at thesame position as the current block. Herein, the block at the sameposition may mean at least one of a block (BR) adjacent to a bottomright corner of the current block, a block (CTR) including a position ofa center sample of the current block, or a block (TL) including aposition of a top left sample of the current block. Alternatively, theblock at the same position may mean a block including a position shiftedby a predetermined disparity vector from a position of a top left sampleof the current block. Herein, the disparity vector may be determinedbased on any one of motion vectors of the spatial neighboring blockdescribed above. Alternatively, the disparity vector may be determinedbased on a combination of at least two of motion vectors of the spatialneighboring blocks described above. The combination may mean anoperation such as a maximum value, a minimum value, a median value, anda weighted average value. For example, a disparity vector may be set asa motion vector of a left neighboring block or a top neighboring block.Alternatively, a disparity vector may be set as a median value or anaverage value between a motion vector of a left neighboring block and amotion vector of a bottom left neighboring block.

A motion vector and a reference picture index of a temporal mergecandidate may be derived from a motion vector and a reference pictureindex of the above-described temporal neighboring block, respectively.Alternatively, a motion vector of a temporal merge candidate may bederived as a motion vector of a temporal neighboring block, and areference picture index of the temporal merge candidate may be set to adefault value (e.g., 0) pre-promised to a decoding apparatus regardlessof the temporal neighboring block.

A merge candidate list may further include a combined merge candidate.The combined merge candidate may be derived by combining n mergecandidates belonging to a pre-generated merge candidate list.

Herein, n may be an integer of 2, 3, 4 or more. The number n of mergecandidates to be combined may be a fixed value pre-promised to anencoding/decoding apparatus, or may be encoded and signaled by anencoding apparatus. The signaling may be performed in at least one unitof a sequence, a picture, a slice, a tile, a sub-tile (brick), or apredetermined block. The number n of merge candidates to be combined maybe variably determined based on the number of remaining mergecandidates. Herein, the number of remaining merge candidates may mean adifference between the maximum number of merge candidates that can beincluded in a merge candidate list and the current number of mergecandidates in the merge candidate list. The maximum number may be anumber pre-promised to an encoding/decoding apparatus, or may be encodedand signaled by an encoding apparatus. The current number may mean thenumber of merge candidates configured before adding a combined mergecandidate. For example, when the number of remaining merge candidates is1, two merge candidates may be used, and when the number of remainingmerge candidates is greater than 1, three or more merge candidates maybe used.

The positions of the n merge candidates may be pre-determined positionsin a merge candidate list. For example, indexes (0 to (k−1)) may beallocated for each merge candidate belonging to a merge candidate list.Herein, k may mean the total number of merge candidates included in amerge candidate list. In this case, the positions of n merge candidatesmay correspond to index 0 to index (n−1) in a merge candidate list.Alternatively, the n merge candidates may be determined in considerationof a prediction direction of each merge candidate included in a mergecandidate list. For example, among merge candidates belonging to a mergecandidate list, only a merge candidate whose prediction direction is abidirectional prediction may be selectively used, or only a mergecandidate whose prediction direction is a unidirectional prediction maybe selectively used.

A combined merge candidate may be derived using both a spatial mergecandidate and a temporal merge candidate, or may be derived using onlyone of a spatial merge candidate or a temporal merge candidate. Forexample, a combined merge candidate may be limited to be derived usingonly spatial merge candidates. In this case, the number of mergecandidates to be combined may be limited within the number of spatialmerge candidates belonging to a pre-generated merge candidate list.

The combined merge candidate may be added after spatial/temporal mergecandidates in a merge candidate list. That is, an index of a combinedmerge candidate may be larger than indices of spatial/temporal mergecandidates. Alternatively, the combined merge candidate may be addedbetween a spatial merge candidate and a temporal merge candidate in amerge candidate list. That is, an index of a combined merge candidatemay be larger than indices of spatial merge candidates and smaller thanindices of temporal merge candidates. Alternatively, a position of acombined merge candidate may be variably determined in consideration ofa prediction direction of the combined merge candidate. Depending onwhether a prediction direction of a combined merge candidate isbidirectional prediction, a position of a combined merge candidate in amerge candidate list may be rearranged. For example, when a predictiondirection of a combined merge candidate is bidirectional prediction, anindex smaller than a spatial or temporal merge candidate may beallocated, otherwise, an index larger than the spatial or temporal mergecandidate may be allocated.

Hereinafter, for convenience of description, a method of deriving acombined merge candidate based on two merge candidates will bedescribed.

Motion information of a combined merge candidate may be derived by aweighted average of motion information of a first merge candidate and asecond merge candidate. Herein, weights of the weighted average are[1:1], [1:2], [1:3], [2:3], etc., but are not limited thereto. Theweight may be pre-defined in an encoding/decoding apparatus or derivedfrom a decoding apparatus. In this case, the weight may be derived byconsidering at least one of a distance between a current picture and areference picture of a merge candidate or a prediction direction of amerge candidate. Alternatively, motion information of a combined mergecandidate may be derived by obtaining motion information in a L0direction from the first merge candidate and motion information in a L1direction from the second merge candidate and combining them. Motioninformation of a combined merge candidate may be derived based on atleast one of the above-described derivation methods, and this may beperformed in consideration of a prediction direction of a mergecandidate to be combined, as described later.

In this specification, motion information may include at least one of aprediction direction flag, a reference picture index, or a motionvector. The motion information may be defined for L0 prediction and L1prediction, respectively. Herein, L0 prediction may mean predictionreferring to a reference picture list L0, and L1 prediction may meanprediction referring to a reference picture list L1.

(1) When prediction directions of a first merge candidate and a secondmerge candidate are both unidirectional prediction,

[CASE 1] When a first merge candidate is L0 prediction and a secondmerge candidate is L1 prediction, a reference picture index of acombined merge candidate in a L0 direction may be derived as a referencepicture index of the first merge candidate. A prediction direction flagin a L0 direction of a combined merge candidate may be derived as 1. Amotion vector of a combined merge candidate in a L0 direction may bederived as a motion vector of a first merge candidate. A referencepicture index of a combined merge candidate in a L1 direction may bederived as a reference picture index of a second merge candidate. Aprediction direction flag in a L1 direction of a combined mergecandidate may be derived as 1. A motion vector of a combined mergecandidate in a L1 direction may be derived as a motion vector of asecond merge candidate.

[CASE 2] When a first merge candidate is L1 prediction and a secondmerge candidate is L0 prediction, a reference picture index of acombined merge candidate in a L0 direction may be derived as a referencepicture index of the second merge candidate. A prediction direction flagin a L0 direction of a combined merge candidate may be derived as 1. Amotion vector of a combined merge candidate in a L0 direction may bederived as a motion vector of a second merge candidate. A referencepicture index of a combined merge candidate in a L1 direction may bederived as a reference picture index of a first merge candidate. Aprediction direction flag in a L1 direction of a combined mergecandidate may be derived as 1. A motion vector of a combined mergecandidate in a L1 direction may be derived as a motion vector of a firstmerge candidate.

[CASE 3] When a first merge candidate and a second merge candidate areL0 prediction, a reference picture index of a combined merge candidatein a L0 direction may be derived as a reference picture index of eitherthe first merge candidate or the second merge candidate. For example, areference picture index of a merge candidate having the smallest indexamong a first merge candidate and a second merge candidate may be set asa reference picture index of a combined merge candidate in a L0direction. A prediction direction flag in a L0 direction of a combinedmerge candidate may be derived as 1. A motion vector of a combined mergecandidate in a L0 direction may be derived as a weighted average of amotion vector of a first merge candidate and a motion vector of a secondmerge candidate. A reference picture index in a L1 direction of acombined merge candidate may be derived as −1, a prediction directionflag in a L1 direction may be derived as 0, and motion information in aL1 direction may be derived as 0.

[CASE 4] When a first merge candidate and a second merge candidate areL1 prediction, a reference picture index in a L0 direction of a combinedmerge candidate may be derived as −1, a prediction direction flag in aL0 direction may be derived as 0, and motion information in a L0direction may be derived as 0. A reference picture index of a combinedmerge candidate in a L1 direction may be derived as a reference pictureindex of either a first merge candidate or a second merge candidate. Forexample, a reference picture index of a merge candidate having thesmallest index among a first merge candidate and a second mergecandidate may be set as a reference picture index of a combined mergecandidate in a L1 direction. A prediction direction flag in a L1direction of a combined merge candidate may be derived as 1. A motionvector of a combined merge candidate in a L1 direction may be derived asa weighted average of a motion vector of a first merge candidate and amotion vector of a second merge candidate.

(2) When prediction directions of a first merge candidate and a secondmerge candidate are both bidirectional prediction,

[CASE 5] A reference picture index of a combined merge candidate in a L0direction may be derived as a reference picture index of either a firstmerge candidate or a second merge candidate. For example, a referencepicture index of a merge candidate having the smallest index among afirst merge candidate and a second merge candidate may be set as areference picture index of a combined merge candidate in a L0 direction.A prediction direction flag in a L0 direction of the combined mergecandidate may be derived as 1. A motion vector of a combined mergecandidate in a L0 direction may be derived as a weighted average of amotion vector of a first merge candidate and a motion vector of a secondmerge candidate. A reference picture index of a combined merge candidatein a L1 direction may be derived as a reference picture index of eithera first merge candidate or a second merge candidate. For example, areference picture index of a merge candidate having the smallest indexamong a first merge candidate and a second merge candidate may be set asa reference picture index of a combined merge candidate in the L1direction. A prediction direction flag in a L1 direction of a combinedmerge candidate may be derived as 1. A motion vector of a combined mergecandidate in a L1 direction may be derived as a weighted average of amotion vector of a first merge candidate and a motion vector of a secondmerge candidate.

(3) When a prediction direction of a first merge candidate isbidirectional prediction and a prediction direction of a second mergecandidate is unidirectional prediction,

[CASE 6] When a second merge candidate is L0 prediction, a referencepicture index of a combined merge candidate in a L0 direction may bederived as a reference picture index of either a first merge candidateor the second merge candidate. For example, a reference picture index ofa merge candidate having the smallest index among a first mergecandidate and a second merge candidate may be set as a reference pictureindex of a combined merge candidate in a L0 direction. A predictiondirection flag in a L0 direction of a combined merge candidate may bederived as 1. A motion vector of a combined merge candidate in a L0direction may be derived as a weighted average of a motion vector of afirst merge candidate and a motion vector of a second merge candidate. Areference picture index of a combined merge candidate in a L1 directionmay be derived as a reference picture index of a first merge candidate.A prediction direction flag in a L1 direction of a combined mergecandidate may be derived as 1. A motion vector of a combined mergecandidate in a L1 direction may be derived as a motion vector of a firstmerge candidate.

[CASE 7] When a second merge candidate is L1 prediction, a referencepicture index in a L0 direction of a combined merge candidate may bederived as a reference picture index of a first merge candidate. Aprediction direction flag in a L0 direction of a combined mergecandidate may be derived as 1. A motion vector of a combined mergecandidate in a L0 direction may be derived as a motion vector of a firstmerge candidate. A reference picture index of a combined merge candidatein a L1 direction may be derived as a reference picture index of eitherthe first merge candidate or a second merge candidate. For example, areference picture index of a merge candidate having the smallest indexamong a first merge candidate and a second merge candidate may be set asa reference picture index of a combined merge candidate in a L1direction. A prediction direction flag in a L1 direction of the combinedmerge candidate may be derived as 1. A motion vector of a combined mergecandidate in a L1 direction may be derived as a weighted average of amotion vector of a first merge candidate and a motion vector of a secondmerge candidate.

Motion information of a current block may be derived from the mergecandidate list (S2).

Specifically, a merge index of a current block may be signaled. A mergeindex may specify any one of a plurality of merge candidates belongingto a merge candidate list. A merge candidate having the same index as amerge index may be extracted, and motion information of a current blockmay be derived using motion information of the extracted mergecandidate. For example, a motion vector, a reference picture index, andprediction direction information of a current block may be set to be thesame as a motion vector, a reference picture index and predictiondirection information of the extracted merge candidate.

2. AMVP Mode

In the case of an AMVP mode, a motion vector of a neighboring block maybe set as a motion vector prediction value of a current block. For thispurpose, a candidate list consisting of motion vectors ofspatial/temporal neighboring blocks may be constructed, and an indexspecifying any one of a plurality of motion vectors of a candidate listmay be signaled. Meanwhile, unlike a merge mode, a motion vector may bereconstructed by adding the motion vector prediction value and asignaled motion vector difference value.

3. Affine Mode

An additional motion model may be used in addition to a translationmotion model that considers only parallel movement. For example, it ispossible to use a motion model that considers motion such as rotation,perspective, and zoom-in/out as well as parallel movement. This will bereferred to as an affine mode. In an affine mode, motion information maybe derived in units of predetermined sub-blocks based on a control pointvector of a current block. Accordingly, an affine mode may be referredto as an inter mode in units of sub-blocks or a merge mode in units ofsub-blocks. This will be described in detail with reference to FIGS. 4to 7 .

4. Intra Block Copy (IBC) Mode

In the case of an IBC mode, it is similar to an AMVP mode in that amotion vector of a neighboring block is set as a block vector predictionvalue (bvp) of a current block, and a block vector is reconstructedusing a signaled block vector difference value (bvd). However, there isa difference in that an IBC mode performs motion compensation based on apre-restored region in the same picture as a current block, whereas anAMVP mode performs motion compensation based on a pre-restored area in apicture different from a current block.

Referring to FIG. 3 , according to a predetermined condition, limitedbidirectional prediction may be performed on a current block (S310).

A predetermined condition may include at least one of a condition for ablock size or a condition on whether to encode in inter mode in units ofsub-blocks. According to a predetermined condition, motion informationfor bidirectional prediction may be adjusted to motion information forunidirectional prediction. This assumes that motion information derivedin S300 is derived for L0 prediction and L1 prediction, respectively,and such limited bidirectional prediction will be described in detailwith reference to FIGS. 9 to 13 .

FIG. 4 illustrates a concept of performing inter prediction using affinetransform prediction in a video coding method and apparatus according toan embodiment of the present disclosure.

The affine transform prediction means a method of performing predictionin an arbitrary shape by generating a motion vector corresponding topixels of a reference picture by using affine transform of a pixel in acurrent block. However, the affine transform prediction is not limitedto prediction in units of pixels, and also refers to a method ofperforming prediction by dividing a current block into a plurality ofsub-blocks to obtain a motion vector in units of sub-blocks inclusively.

In a motion prediction method based on affine transformation prediction,in performing prediction with a reference block inside a referencepicture 210 for a current block 201 inside a current picture 200, two ormore positions representing the block are selected as control points,and rotation and motion prediction of an arbitrary shape are performedusing two or more control motion vectors 230 and 231 for the controlpoints. In this case, it includes a method of performing pixel-by-pixelmotion prediction by calculating a motion vector for every pixel in acurrent block 201 using a plurality of control motion vectors, or amethod of performing sub-block unit motion prediction by dividing acurrent block 201 into two or more sub-blocks and calculating a motionvector in units of the sub-blocks.

FIG. 5 illustrates a concept of affine transform prediction in units ofsub-blocks according to an embodiment of the present disclosure.

FIG. 3 is a diagram illustrating an embodiment in which one block isdivided into two or more sub-blocks, and inter prediction is performedusing each motion vector in units of the sub-blocks. According to anembodiment of the present disclosure, a method of selecting two or morecontrol points representing a current block 300 and transforming andobtaining motion vectors for each of two or more sub-blocks dividing thecurrent block 300 using a motion vector corresponding to the controlpoints. In this case, the meaning of transforming the motion vector mayalso be used as a meaning of calculating the motion vector.

According to an embodiment of the present disclosure, it includes amethod of selecting two or more control points representing a currentblock 300 and transforming and obtaining each motion vector for two ormore sub-blocks dividing a current block 300 by using a motion vectorcorresponding to the control points. The control points representing acurrent block 300 may be two points of a top left position 311 and a topright position 321, or may be three points of a top left position 311, atop right position 321, and a bottom left position 331. In addition, acenter position of a block and a bottom right position of a block may beused as control points, and it is also possible to use two or morepoints as control points.

According to an embodiment of the present disclosure, according to anequation having a motion vector corresponding to the control points as aparameter, motion prediction for each sub-block is performed bycalculating a motion vector for each sub-block dividing a current block300.

In addition, as a method of dividing a current block 300 into two ormore sub-blocks, there is a method of dividing into square blocks inwhich N and M having a predefined fixed size or non-square blocks inwhich N and M are different from each other. Additionally, a method oftransmitting a size of a sub-block using a higher-level syntax may alsobe used. In addition, a method of calculating a size of a sub-blockusing a motion vector corresponding to control points representing acurrent block 300 is also included.

FIG. 6 illustrates an affine mode based inter prediction method as anembodiment to which the present disclosure is applied.

Referring to FIG. 6 , a candidate list for predicting motion informationof a current block may be generated (S600).

The candidate list may include one or more affine mode based candidates(hereinafter, referred to as affine candidates). An affine candidate maymean a candidate having a control point vector. A control point vectormay mean a motion vector of a control point for an affine mode, and maybe defined for a corner position of a block (e.g., at least one of a topleft corner, a top right corner, a bottom left corner, or a bottom rightcorner).

An affine candidate may include at least one of a spatial candidate, atemporal candidate, or a configured candidate. Herein, the spatialcandidate may be derived from a vector of a neighboring block spatiallyadjacent to a current block, and the temporal candidate may be derivedfrom a vector of a neighboring block temporally adjacent to a currentblock. Herein, the neighboring block may mean a block encoded in anaffine mode. The vector may mean a motion vector or a control pointvector.

1. Spatial/temporal candidate derivation method

A width and a height of a current block 500 are cbW and cbH,respectively, and a position of a current block is (xCb, yCb). A widthand a height of spatial neighboring blocks 510-550 are nbW and nbH,respectively, and a position of the spatial neighboring block are (xNb,yNb). A spatial neighboring block may include at least one of a leftblock 510, a bottom left block 540, a top right block 430, a top block420, or a top left block 450 of a current block. Alternatively, thespatial neighboring block may further include at least one of a blockadjacent to a right of the top left block 450 or a block adjacent to abottom of the top left block 450.

A spatial candidate may have n control point vectors (cpMV). Herein, then value may be an integer of 1, 2, 3, or more. Then value may bedetermined based on at least one of information on whether to be decodedin units of sub-blocks, information on whether a block is encoded in anaffine mode, or information on a type (4-parameter or 6-parameter) of anaffine mode.

For example, according to the information, when a corresponding block isdecoded in units of sub-blocks or is a block encoded in an affine mode,the corresponding block may have two control point vectors. On the otherhand, if not, the corresponding block may not perform affine mode basedprediction.

Alternatively, according to the information, when a corresponding blockis a block encoded in an affine mode, and a type of the affine mode is6-parameter, the corresponding block may have 3 control point vectors.On the other hand, if not, the corresponding block may not performaffine mode based prediction.

The above-described information may be encoded and signaled by anencoding apparatus. Alternatively, all or part of the information may bederived from a decoding apparatus based on properties of a block.Herein, the block may mean a current block or a spatial/temporalneighboring block of a current block. The properties may mean a size, ashape, a position, a division type, an inter mode, a parameter relatedto a residual coefficient, and the like. The inter mode is a modepre-defined in a decoding apparatus and may mean a merge mode, a skipmode, an AMVP mode, an affine mode, an intra/inter combination mode, anIBC mode, and the like. Alternatively, a value of n may be derived froma decoding apparatus based on the above-described block properties.

In this embodiment, n control point vectors may be expressed as a firstcontrol point vector (cpMV[0]), a second control point vector (cpMV[1]),a third control point vector (cpMV[2]), . . . an n-th control pointvector (cpMV[n−1]).

As an example, a first control point vector (cpMV[0]), a second controlpoint vector (cpMV[1]), a third control point vector (cpMV[2]), and afourth control point vector (cpMV[3]) may be a vector corresponding tothe positions of a top left sample, a top right sample, a bottom leftsample, and a bottom right sample of a block, respectively. Herein, itis assumed that a spatial candidate have three control point vectors,and the three control point vectors may be arbitrary control pointvectors selected from the first to n-th control point vectors. However,the present disclosure is not limited thereto, and a spatial candidatemay have two control point vectors, and the two control point vectorsmay be arbitrary control point vectors selected from the first to n-thcontrol point vectors.

A control point vector of a spatial candidate may be derived inconsideration of whether a boundary of a current block contacts a CTUboundary.

(1) When a boundary of a current block does not contact a CTU boundary,

The first control point vector may be derived based on at least one of afirst control point vector of a spatial neighboring block, apredetermined difference value, position information of a current block(xCb, yCb), or position information of a spatial neighboring block (xNb,yNb).

The number of difference values may be 1, 2, 3 or more. The number ofthe difference values may be variably determined in consideration of theproperties of the above-described block, or may be a fixed valuepre-promised to a decoding apparatus. The difference value may bedefined as a difference value between one of a plurality of controlpoint vectors and the other. For example, the difference value mayinclude a first difference value between the second control point vectorand the first control point vector, a second difference value betweenthe third control point vector and the first control point vector, athird difference value between the fourth control point vector and thethird control point vector, or a fourth difference value between thefourth control point vector and the second control point vector.

For example, the first control point vector may be derived as inEquation 1 below.

[Equation 1]cpMvLX[0][0]=(mvScaleHor+dHorX*(xCb−xNb)+dHorY*(yCb−yNb))cpMvLX[0][1]=(mvScaleVer+dVerX*(xCb−xNb)+dVerY*(yCb−yNb))

In Equation 1, the variables mvScaleHor and mvScaleVer may mean a firstcontrol point vector of a spatial neighboring block, or a value derivedby applying a shift operation by k to the first control point vector.Herein, k may be an integer of 1, 2, 3, 4, 5, 6, 7, 8, 9 or more. Thevariables dHorX and dVerX correspond to x and y components of the firstdifference value between the second control point vector and the firstcontrol point vector, respectively. The variables dHorY and dVerYcorrespond to x and y components of the second difference value betweenthe third control point vector and the first control point vector,respectively. The above-described variable may be derived as in Equation2 below.

[Equation 2]mvScaleHor=CpMvLX[xNb][yNb][0][0]<<7mvScaleVer=CpMvLX[xNb][yNb][0][1]<<7dHorX=(CpMvLX[xNb+nNbW−1][yNb][1][0]−CpMvLX[xNb][yNb][0][0])<<(7−log2NbW)dVerX=(CpMvLX[xNb+nNbW−1][yNb][1][1]−CpMvLX[xNb][yNb][0][1])<<(7−log2NbW)dHorY=(CpMvLX[xNb][yNb+nNbH−1][2][0]−CpMvLX[xNb][yNb][2][0])<<(7−log2NbH)dVerY=(CpMvLX[xNb][yNb+nNbH−1][2][1]−CpMvLX[xNb][yNb][2][1])<<(7−log2NbH)

The second control point vector may be derived based on at least one ofa first control point vector of a spatial neighboring block, apredetermined difference value, position information of a current block(xCb, yCb), a block size (width or height), or position information of aspatial neighboring block (xNb, yNb). Herein, the block size may mean asize of a current block and/or a spatial neighboring block. Thedifference value is as described in the first control point vector, anda detailed description thereof will be omitted. However, a range and/ornumber of difference values used in a process of deriving the secondcontrol point vector may be different from the first control pointvector.

For example, the second control point vector may be derived as inEquation 3 below.

[Equation 3]cpMvLX[1][0]=(mvScaleHor+dHorX*(xCb+cbWidth−xNb)+dHorY*(yCb−yNb))cpMvLX[1][1]=(mvScaleVer+dVerX*(xCb+cbWidth−xNb)+dVerY*(yCb−yNb))

In Equation 3, the variables mvScaleHor, mvScaleVer, dHorX, dVerX,dHorY, and dVerY are as described in Equation 1, and detaileddescriptions thereof will be omitted.

The third control point vector may be derived based on at least one of afirst control point vector of a spatial neighboring block, apredetermined difference value, position information of a current block(xCb, yCb), a block size (width or height), or position information of aspatial neighboring block (xNb, yNb). Herein, the block size may mean asize of a current block and/or a spatial neighboring block. Thedifference value is as described in the first control point vector, anda detailed description thereof will be omitted here. However, a rangeand/or number of difference values used in a process of deriving thethird control point vector may be different from the first control pointvector or the second control point vector.

For example, the third control point vector may be derived as inEquation 4 below.

[Equation 4]cpMvLX[2][0]=(mvScaleHor+dHorX*(xCb−xNb)+dHorY*(yCb+cbHeight−yNb))cpMvLX[2][1]=(mvScaleVer+dVerX*(xCb−xNb)+dVerY*(yCb+cbHeight−yNb))

In Equation 4, the variables mvScaleHor, mvScaleVer, dHorX, dVerX,dHorY, and dVerY are as described in Equation 1, and detaileddescriptions thereof will be omitted. Meanwhile, through theabove-described process, an n-th control point vector of a spatialcandidate may be derived.

(2) When a boundary of a current block contacts a CTU boundary,

The first control point vector may be derived based on at least one of amotion vector (MV) of a spatial neighboring block, a predetermineddifference value, position information of a current block (xCb, yCb), orposition information of a spatial neighboring block (xNb, yNb).

The motion vector may be a motion vector of a sub-block located at abottom of a spatial neighboring block. The sub-block may be located atthe leftmost, center, or rightmost among a plurality of sub-blockslocated at a bottom of a spatial neighboring block. Alternatively, themotion vector may mean an average value, a maximum value, or a minimumvalue of motion vectors of a sub-blocks.

The number of difference values may be 1, 2, 3 or more. The number ofthe difference values may be variably determined in consideration of theproperties of the above-described block, or may be a fixed valuepre-promised to a decoding apparatus. The difference value may bedefined as a difference value between one of a plurality of motionvectors stored in a unit of a sub-block in a spatial neighboring blockand the other. For example, the difference value may mean a differencevalue between a motion vector of a bottom right sub-block and a motionvector of a bottom left sub-block of a spatial neighboring block.

For example, the first control point vector may be derived as inEquation 5 below.

[Equation 5]cpMvLX[0][0]=(mvScaleHor+dHorX*(xCb−xNb)+dHorY*(yCb−yNb))cpMvLX[0][1]=(mvScaleVer+dVerX*(xCb−xNb)+dVerY*(yCb−yNb))

In Equation 5, the variables mvScaleHor and mvScaleVer may mean a motionvector (MV) of a spatial neighboring block described above or a valuederived by applying a shift operation by k to the motion vector. Herein,k may be an integer of 1, 2, 3, 4, 5, 6, 7, 8, 9 or more.

The variables dHorX and dVerX correspond to x and y components of apredetermined difference value, respectively. Herein, the differencevalue means a difference value between a motion vector of a bottom rightsub-block and a motion vector of a bottom left sub-block in a spatialneighboring block. The variables dHorY and dVerY may be derived based onthe variables dHorX and dVerX. The above-described variable may bederived as in Equation 6 below.

[Equation 6]mvScaleHor=MvLX[xNb][yNb+nNbH−1][0]<<7mvScaleVer=MvLX[xNb][yNb+nNbH−1][1]<<7dHorX=(MvLX[xNb+nNbW−1][yNb+nNbH−1][0]−MvLX[xNb][yNb+nNbH−1][0])<<(7−log2NbW)dVerX=(MvLX[xNb+nNbW−1][yNb+nNbH−1][1]−MvLX[xNb][yNb+nNbH−1][1])<<(7−log2NbW)dHorY=−dVerXdVerY=−dHorX

The second control point vector may be derived based on at least one ofa motion vector (MV) of a spatial neighboring block, a predetermineddifference value, position information of a current block (xCb, yCb), ablock size (width or height), or position information of spatialneighboring block (xNb, yNb). Herein, the block size may mean a size ofa current block and/or a spatial neighboring block. The motion vectorand the difference value are as described in the first control pointvector, and a detailed description thereof will be omitted. However, aposition of a motion vector used in a process of deriving the secondcontrol point vector, a range and/or the number of difference values maybe different from the first control point vector.

For example, the second control point vector may be derived as inEquation 7 below.

[Equation 7]cpMvLX[1][0]=(mvScaleHor+dHorX*(xCb+cbWidth−xNb)+dHorY*(yCb−yNb))cpMvLX[1][1]=(mvScaleVer+dVerX*(xCb+cbWidth−xNb)+dVerY*(yCb−yNb))

In Equation 7, the variables mvScaleHor, mvScaleVer, dHorX, dVerX,dHorY, and dVerY are as described in Equation 5, and detaileddescriptions thereof will be omitted.

The third control point vector may be derived based on at least one of amotion vector (MV) of a spatial neighboring block, a predetermineddifference value, position information of a current block (xCb, yCb), ablock size (width or height), or position information of spatialneighboring block (xNb, yNb). Herein, the block size may mean a size ofa current block and/or a spatial neighboring block. The motion vectorand the difference value are as described in the first control pointvector, and detailed descriptions thereof will be omitted. However, aposition of a motion vector used in a process of deriving the thirdcontrol point vector, a range and/or the number of difference values maybe different from the first control point vector or the second controlpoint vector.

For example, the third control point vector may be derived as inEquation 8 below.

[Equation 8]cpMvLX[2][0]=(mvScaleHor+dHorX*(xCb−xNb)+dHorY*(yCb+cbHeight−yNb))cpMvLX[2][1]=(mvScaleVer+dVerX*(xCb−xNb)+dVerY*(yCb+cbHeight−yNb))

In Equation 8, the variables mvScaleHor, mvScaleVer, dHorX, dVerX,dHorY, and dVerY are as described in Equation 5, and detaileddescriptions thereof will be omitted. Meanwhile, through theabove-described process, the n-th control point vector of a spatialcandidate may be derived.

The above-described process of deriving the affine candidate may beperformed for each pre-defined spatial neighboring block. Thepre-defined spatial neighboring block may include at least one of a leftblock, a bottom left block, a top right block, a top block, or a topleft block of a current block.

Alternatively, a process of deriving an affine candidate may beperformed for each group of spatial neighboring blocks. Herein, thespatial neighboring blocks may be classified into a first groupincluding a left block and a bottom left block, and a second groupincluding a top right block, a top block, and a top left block.

For example, one affine candidate may be derived from spatialneighboring blocks belonging to the first group. The derivation may beperformed until an available affine candidate is found based on apredetermined priority. The priority may be an order of a left block→abottom left block, or the reverse order. According to the priority, itis determined whether a corresponding spatial neighboring block in thefirst group is a block decoded through affine mode based prediction, anda block decoded by the very first affine mode based prediction may beselected as an affine candidate.

Likewise, one affine candidate may be derived from a spatial neighboringblock belonging to the second group. The derivation may be performeduntil an available affine candidate is found based on a predeterminedpriority. The priority may be in an order of a top right block→a topblock→a top left block, or the reverse order. According to the priority,it is determined whether a corresponding spatial neighboring block inthe second group is a block decoded through affine mode basedprediction, and a block decoded by the very first affine mode basedprediction may be selected as an affine candidate.

The above-described embodiment may be applied in the same/similar mannerto a temporal neighboring block. Herein, the temporal neighboring blockmay belong to a picture different from a current block, and may be ablock at the same position as a current block. The block at the sameposition may be a block including a position of a top left sample, acenter position, or a position of a sample adjacent to a bottom rightsample of a current block.

Alternatively, a temporal neighboring block may mean a block at aposition shifted by a predetermined disparity vector from the block atthe same position. Herein, the disparity vector may be determined basedon a motion vector of any one of the above-described spatial neighboringblocks of the current block.

2. A method of deriving a configured candidate

A configured candidate may be derived based on a combination betweenmotion vectors of spatial/temporal neighboring blocks in a currentblock. A configured candidate may be derived based on a combination ofat least two of control point vectors (hereinafter, referred to ascontrol point vectors (cpMVCorner[n])) corresponding to each corner of acurrent block. Herein, then may be 0, 1, 2, or 3.

The control point vector (cpMVCorner[n]) may be derived based on amotion vector of a spatial neighboring block and/or a temporalneighboring block. Herein, the spatial neighboring block may include atleast one of a first neighboring block (A, B or C) adjacent to a topleft sample of a current block, a second neighboring block (D or E)adjacent to a top right sample of the current block, or a thirdneighboring block (F or G) adjacent to a bottom left sample of thecurrent block. The temporal neighboring block may be a block belongingto a picture different from the current block, and may mean a block atthe same position as the current block (hereinafter, referred to as afourth neighboring block Col). Herein, the fourth neighboring block maymean a block (H, I, or J) including a position of a top left sample, atop right sample, or a bottom left sample of the current block, or ablock adjacent to a position of a bottom right sample of the currentblock.

The first neighboring block may mean a neighboring block at a top left(A), top (B), or left (C) of a current block. It is determined whethermotion vectors of neighboring blocks A, B, and C are available accordingto a predetermined priority, and a control point vector may bedetermined using motion vectors of available neighboring blocks. Theavailability determination may be performed until a neighboring blockhaving an available motion vector is found. Herein, the priority may bein an order of A→B→C. However, the present disclosure is not limitedthereto, and may be in an order of A→C→B, C→A→B, or B→A→C.

The second neighboring block may mean a top neighboring block (D) or atop right neighboring block (E) of a current block. Likewise, whethermotion vectors of neighboring blocks D and E are available may bedetermined according to a predetermined priority, and a control pointvector may be determined by using motion vectors of availableneighboring blocks. The availability determination may be performeduntil a neighboring block having an available motion vector is found.Herein, the priority may be in an order of D→E, or in an order of E→D.

The third neighboring block may mean a left neighboring block (F) or abottom left neighboring block (G) of a current block. Likewise, whethera motion vector of a neighboring block is available may be determinedaccording to a predetermined priority, and a control point vector may bedetermined by using the motion vector of the neighboring block that isavailable. The availability determination may be performed until aneighboring block having an available motion vector is found. Herein,the priority may be in an order of G→F, or in an order of F→G.

For example, a first control point vector (cpMVCorner[0]) may be set asa motion vector of a first neighboring block, a second control pointvector (cpMVCorner[1]) may be set as a motion vector of a secondneighboring block, and a third control point vector cpMVCorner[2] may beset as a motion vector of a third neighboring block. A fourth controlpoint vector cpMVCorner[3] may be set as a motion vector of a fourthneighboring block.

Alternatively, a first control point vector may be derived using atleast one motion vector of a first neighboring block or a fourthneighboring block, wherein the fourth neighboring block may be a block(H) including a position of a top left sample. A second control pointvector may be derived using at least one motion vector of a secondneighboring block or a fourth neighboring block, wherein the fourthneighboring block may be a block (I) including a position of a top rightsample. A third control point vector may be derived using at least onemotion vector of a third neighboring block or a fourth neighboringblock, wherein the fourth neighboring block may be a block (J) includinga position of a bottom left sample.

Alternatively, any one of first to fourth control point vectors may bederived based on the other. For example, a second control point vectormay be derived by applying a predetermined offset vector to a firstcontrol point vector. The offset vector may be a difference vectorbetween a third control point vector and a first control point vector,or may be derived by applying a predetermined scaling factor to thedifference vector. The scaling factor may be determined based on atleast one of a width or height of a current block and/or a neighboringblock.

K configured candidates ConstK according to the present disclosure maybe determined through a combination of at least two of first to fourthcontrol point vectors described above. The K value may be an integer of1, 2, 3, 4, 5, 6, 7 or more. The K value may be derived based oninformation signaled by an encoding apparatus or may be a valuepre-promised to a decoding apparatus. The information may includeinformation indicating the maximum number of configured candidatesincluded in a candidate list.

Specifically, a first configured candidate Const1 may be derived bycombining first to third control point vectors. For example, a firstconfigured candidate Const1 may have a control point vector as shown inTable 1 below. Meanwhile, only when reference picture information of afirst neighboring block is the same as reference picture information ofsecond and third neighboring blocks, a control point vector may beconfigured as shown in Table 1. Herein, the reference pictureinformation may mean a reference picture index indicating a position ofa corresponding reference picture in a reference picture list, or apicture order count (POC) value indicating an output order.

TABLE 1 Idx Control point vector 0 cpMvCorner[ 0 ] 1 cpMvCorner[ 1 ] 2cpMvCorner[ 2 ]

A second configured candidate Const2 may be derived by combining first,second, and fourth control point vectors. For example, a secondconfigured candidate Const2 may have a control point vector as shown inTable 2 below. Meanwhile, only when reference picture information of afirst neighboring block is the same as reference picture information ofsecond and fourth neighboring blocks, a control point vector may beconfigured as shown in Table 2. Herein, the reference pictureinformation is as described above.

TABLE 2 Idx Control point vector 0 cpMvCorner[ 0 ] 1 cpMvCorner[ 1 ] 2cpMvCorner[ 3 ] + cpMvCorner[ 1 ] − cpMvCorner[ 0 ] cpMvCorner[ 3 ] +cpMvCorner[ 0 ] − cpMvCorner[ 1 ]

A third configured candidate Const3 may be derived by combining first,third, and fourth control point vectors. For example, a third configuredcandidate Const3 may have a control point vector as shown in Table 3below. Meanwhile, only when reference picture information of a firstneighboring block is the same as reference picture information of thirdand fourth neighboring blocks, a control point vector may be configuredas shown in Table 3. Herein, the reference picture information is asdescribed above.

TABLE 3 Idx Control point vector Control point vector 0 cpMvCorner[0]cpMvCorner[0] 1 cpMvCorner[3] + cpMvCorner[2] cpMvCorner[0] −cpMvCorner[2] 2 cpMvCorner[2] cpMvCorner[0] + cpMvCorner[3] −cpMvCorner[2]

A fourth configured candidate Const4 may be derived by combining second,third, and fourth control point vectors. For example, a fourthconfigured candidate Const4 may have a control point vector as shown inTable 4 below. Meanwhile, only when reference picture information of asecond neighboring block is the same as reference picture information ofthird and fourth neighboring blocks, a control point vector may beconfigured as shown in Table 4. Herein, the reference pictureinformation is as described above.

TABLE 4 Idx Control point vector Control point vector 0 cpMvCorner[1] +cpMvCorner[2] cpMvCorner[2] − cpMvCorner[3] 1 cpMvCorner[1]cpMvCorner[3] 2 cpMvCorner[2] cpMvCorner[3] + cpMvCorner[2] −cpMvCorner[1]

A fifth configured candidate Const5 may be derived by combining firstand second control point vectors. For example, a fifth configuredcandidate Const5 may have a control point vector as shown in Table 5below. Meanwhile, only when reference picture information of a firstneighboring block is the same as reference picture information of asecond neighboring block, a control point vector may be configured asshown in Table 5. Herein, the reference picture information is asdescribed above.

TABLE 5 Idx Control point vector 1 cpMvCorner[ 0 ] 2 cpMvCorner[ 1 ]

A sixth configured candidate Const6 may be derived by combining firstand third control point vectors. For example, a sixth configuredcandidate Const6 may have a control point vector as shown in Table 6below. Meanwhile, only when reference picture information of a firstneighboring block is the same as reference picture information of athird neighboring block, a control point vector may be configured asshown in Table 6. Herein, the reference picture information is asdescribed above.

TABLE 6 Idx Control point vector Control point vector 1 cpMvCorner[ 0 ]cpMvCorner[ 0 ] 2 cpMvCorner[ 2 ] cpMvCorner[ 1 ]

In Table 6, cpMvCorner[1] may be a second control point vector derivedbased on first and third control point vectors. The second control pointvector may be derived based on at least one of a first control pointvector, a predetermined difference value, or a size of acurrent/neighboring block. For example, a second control point vectormay be derived as in Equation 9 below.

[Equation 9]cpMvCorner[1][0]=(cpMvCorner[0][0]<<7)+((cpMvCorner[2][1]−cpMvCorner[0][1])<<(7+Log2(cbHeight/cbWidth)))cpMvCorner[1][1]=(cpMvCorner[0][1]<<7)+((cpMvCorner[2][0]−cpMvCorner[0][0])<<(7+Log2(cbHeight/cbWidth)))

In the above-described K configured candidates (ConstK), the K valuedoes not limit a position or priority of a configured candidate arrangedin the candidate list.

In addition, all of first to sixth configured candidates may be includedin a candidate list, or only some of the first to sixth configuredcandidates may be included in the candidate list.

For example, when it is determined that a current block uses threecontrol point vectors, only configured candidates generated through acombination of three of first to fourth control point vectors may beused. When it is determined that a current block uses two control pointvectors, a configured candidate generated through a combination of atleast two of first to fourth control point vectors may be used, or aconfigured candidate generated through only a combination of two offirst to fourth control point vectors may be used.

Alternatively, only partially configured candidates may be included in acandidate list in consideration of the maximum number of affinecandidates included in the candidate list. Herein, the maximum numbermay be determined based on information on the maximum number signaled byan encoding apparatus, or may be variably determined in consideration ofthe above-described properties of the current block. In this case, Kvalue of a configured candidate (ConstK) may mean a priority to beinserted into a candidate list.

Meanwhile, when a difference value between two control point vectorsbelonging to the configured candidate is less than a predeterminedthreshold value, the configured candidate may not be inserted into acandidate list. The difference value between the two control pointvectors may be divided into a difference value in a horizontal directionand a difference value in a vertical direction. Herein, the differencevalue in the horizontal direction may mean a difference value between afirst control point vector 610 and a second control point vector 620,and the difference value in the vertical direction may mean a differencevalue between a first control point vector 610 and a third control pointvector 630. The threshold value may mean 0 or a vector having a sizepre-promised to an encoding/decoding apparatus.

The plurality of the affine candidates described above may be arrangedin the candidate list based on a predetermined priority. For example, aplurality of affine candidates may be arranged in a candidate list in anorder of a spatial candidate, a temporal candidate, and a configuredcandidate. Alternatively, a plurality of affine candidates may bearranged in a candidate list in an order of a temporal candidate, aspatial candidate, and a configured candidate. However, the presentdisclosure is not limited thereto, and a temporal candidate may bearranged after a configured candidate. Alternatively, some of configuredcandidates may be arranged before a spatial candidate, and the rest maybe arranged after the spatial candidate.

Referring to FIG. 6 , a control point vector of a current block may bederived based on a candidate list and a candidate index (S610).

A candidate index may mean an index that is coded to derive a controlpoint vector of a current block. The candidate index may specify any oneof a plurality of affine candidates included in a candidate list. Acontrol point vector of a current block may be derived by using acontrol point vector of an affine candidate specified by the candidateindex.

For example, it is assumed that a type of an affine mode of a currentblock is 4-parameter (i.e., it is determined that the current block usestwo control point vectors). In this case, when an affine candidatespecified by the candidate index has three control point vectors, amongthe three control point vectors, only two control point vectors (e.g.,control point vectors with Idx=0, 1) may be selected and set as controlpoint vectors of a current block. Alternatively, the three control pointvectors of a specified affine candidate may be set as control pointvectors of a current block. In this case, a type of an affine mode of acurrent block may be updated to 6-parameter.

Conversely, it is assumed that a type of an affine mode of a currentblock is 6-parameter (i.e., it is determined that the current block uses3 control point vectors). In this case, when an affine candidatespecified by the candidate index has two control point vectors, oneadditional control point vector may be generated, and the two controlpoint vectors of the affine candidate and the additional control pointvector may be used as control point vectors of the current block. Theadditional control point vector may be derived based on at least one oftwo control point vectors of an affine candidate, a size of acurrent/neighboring block, or position information.

Alternatively, two control point vectors of the specified affinecandidate may be set as control point vectors of a current block. Inthis case, a type of an affine mode of a current block may be updated to4-parameter.

Referring to FIG. 6 , a motion vector of a current block may be derivedbased on a control point vector of a current block (S620).

The motion vector may be derived in units of sub-blocks of a currentblock. To this end, the current block may be divided into a plurality ofN×M sub-blocks. Herein, the N×M sub-block may be in a shape of anon-square (N>M or N<M) or a square (N=M). The values of N and M may be4, 8, 16, 32 or more.

In consideration of consumption of a memory bandwidth, a motion vectormay be derived for each sub-block in a current block, or one motionvector may be derived for the current block. This selective derivationmay be performed in consideration of at least one of whether a currentblock performs bidirectional prediction (PRED_BI) or a size of apredetermined reference region (refSize). Herein, the reference regionmay mean a region referenced for motion compensation of one or moresub-blocks. The reference region may include a reference block specifiedby a motion vector of one or more sub-blocks and/or a pixel used tointerpolate a minority pel of a reference block. A size of the referencearea may be expressed by any one of a width or a height of the referencearea, the maximum/minimum value of the width and the height, a productof the width and the height, a sum of the width and the height, and thelike.

For example, when a current block performs bidirectional prediction, anda size of a reference region for n sub-blocks is less than or equal to apredetermined first threshold value (condition 1), a motion vector maybe derived for each sub-block in a current block.

Alternatively, when a current block performs unidirectional prediction,and a size of a reference region for m sub-blocks is less than or equalto a predetermined second threshold value (condition 2), a motion vectormay be derived for each sub-block in a current block.

Then and m values may be 1, 2, 3, 4, 5 or more. The values of n and mmay be limited to even numbers. n may be set to a value larger than m.For example, n may be set to 4 and m may be set to 2.

The first threshold value may be determined as (15+i)×(15+j), and thesecond threshold value may be determined as (15+i)×(11+j) or(11+i)×(15+j). Herein, i and j may be natural numbers greater than orequal to 0. For example, when i and j are 0, the first threshold valueand the second threshold value may be determined as 225 and 165,respectively.

A size of the reference region may be determined using at least one of ascaling value (mvScaleHor, mvScaleVer), a first difference value (dHorX,dVerX), or a second difference value (dHorY, dVerY). A method ofderiving the scaling value, the first difference value, and the seconddifference value is as described above, and a detailed descriptionthereof will be omitted.

A size of the reference region is determined for n or m sub-blocks,respectively, and may be determined as in Equation 10 below inconsideration of an arrangement of n or m sub-blocks.

[Equation 10]maxW4=Max(0, Max(4*(2048+dHorX), Max(4*dHorY, 4*(2048+dHorX)+4*dHorY)))minW4=Min(0, Min(4*(2048+dHorX), Min(4*dHorY, 4*(2048+dHorX)+4*dHorY)))maxH4=Max(0, Max(4*dVerX, Max(4*(2048+dVerY), 4*dVerX+4*(2048+dVerY))))minH4=Min(0, Min(4*dVerX, Min(4*(2048+dVerY), 4*dVerX+4*(2048+dVerY))))bxWX4=((maxW4−minW4)>>11)+9bxHX4=((maxH4−minH4)>>11)+9bxWXh=((Max(0, 4*(2048+dHorX))Min(0,4*(2048+dHorX)))>>11)+9bxHXh=((Max(0, 4*dVerX)−Min(0, 4*dVerX))>>11)+9bxWXv=((Max(0, 4*dHorY)−Min(0, 4*dHorY))>>11)+9bxHXv=((Max(0, 4*(2048+dVerY))−Min(0, 4*(2048+dVerY)))>>11)+9

In Equation 10, bxWX4 and bxHX4 may mean a width and a height of areference region for four sub-blocks, respectively. The four sub-blocksmay be arranged in a 2×2 form. However, the present disclosure is notlimited thereto, and they may be arranged in a 1×4 or 4×1 form. bxWX4and bxHX4 may be derived using a first difference value (dHorX anddVerX) and a second difference value (dHorY and dVerY).

Meanwhile, bxWXh and bxHXh may mean a width and a height of a referencearea for two sub-blocks, respectively. The two sub-blocks may bearranged in a 2×1 form (arranged in a horizontal direction). The bxWXhand the bxHXh may be derived using only a first difference value (dHorXand dVerX). bxWXv and bxHXv may mean a width and a height of a referenceregion for two sub-blocks, respectively. The two sub-blocks may bearranged in a 1×2 form (arranged in a vertical direction). The bxWXv andthe bxHXv may be derived using only a second difference values (dHorYand dVerY).

However, in the case of bidirectional prediction, a size of a referenceregion is determined based on n sub-blocks, and in this case, the nvalue is not limited to 4, and a sub-block may be a square in a 3×3 or4×4 form, or a non-square in a 2×3 or 3×2 form. In addition, in the caseof unidirectional prediction, a size of a reference region is determinedbased on m sub-blocks, and in this case, the m value is not limited to2, and a sub-block may be a square in a 2×2 or 3×3 form, or a non-squarein a 1×4 or 2×4 form. In addition, in the case of unidirectionalprediction, a size of a reference region is determined based on msub-blocks, and in this case, the m value is not limited to 2, and asub-block may be a square in a 2×2 or 3×3 form, or a non-square in a 1×4or 2×4 form.

If the above-described condition 1 or condition 2 is not satisfied, onemotion vector may be derived for a current block. That is, allsub-blocks belonging to a current block may share one and the samemotion vector, thereby reducing memory bandwidth consumption.

Based on a control point vector of a current block, an affine motionparameter may be calculated, and a motion vector of the current blockmay be derived based on the calculated affine motion parameter. Herein,the affine motion parameter may include at least one of theabove-described scaling value, the above-described first differencevalue, or the above-described second difference value.

FIG. 7 illustrates a concept of limiting a motion vector inconsideration of a memory bandwidth as an embodiment to which thepresent disclosure is applied.

When one current block has a plurality of motion vectors, based on oneor more reference motion vectors, properties of remaining motion vectorsmay be limited. The reference motion vector means a motion vectorcorresponding to a top left control point. However, the presentdisclosure is not limited thereto, and the reference motion vector maybe defined as a motion vector corresponding to a bottom left or topright control point. Through this, it is possible to reduce a memorybandwidth for performing motion prediction and compensation. Herein,properties of a motion vector may include at least one of a direction, asize, a number, or a position of the motion vector. Through the abovelimitation, remaining motion vectors may be set as vectors having thesame or similar direction and size as a reference motion vector. Thatis, when it is determined that excessive memory bandwidth is consumeddue to a use of motion vectors in units of sub-blocks, a plurality ofsub-blocks belonging to a current block may be set to share/use onemotion vector.

In FIG. 5 , an embodiment in which a motion vector for each sub-block iscalculated by dividing one block into two or more sub-blocks, selectingtwo or more control points representing one block, and using motionvectors corresponding to corresponding control points is illustrated.

In addition, FIG. 7 shows a concept of limiting remaining motion vectorsbased on a first motion vector among motion vectors corresponding to twoor more control points 311, 321, 331 representing a current block 300.

As shown in FIG. 7 , according to an embodiment of the presentdisclosure, when there are motion vectors 310, 320, 330 corresponding tothree control points 311, 321, 331 of a current block 300, informationsuch as a direction and size of a second motion vector 320 and/or athird motion vector 330 may be changed to be the same/similar to a firstmotion vector by using information such as a direction and size of thefirst motion vector 310. The change of the motion vector refers toclipping or calculating a motion vector to specific values to change avalue of the motion vector.

In this case, in the case of using a method of limiting remaining motionvectors by using a first motion vector, a step of recalculating a sizeand direction of all or part of the remaining motion vectors may beadditionally included using a size and direction of the first motionvector.

Limiting remaining motion vectors by using a first motion vector maymean changing a size and direction of a second motion vector and/or athird motion vector to refer to a position within a predetermined areaspecified by the first motion vector.

FIG. 7 shows an example in which a second motion vector 320 and a thirdmotion vector 330 are changed according to a size and direction of afirst motion vector 310.

FIG. 8 illustrates an inter prediction method using mask-basedprediction in a video coding method and apparatus according to anembodiment of the present disclosure.

A method of limiting a second motion vector and other motion vectorsusing a first motion vector may also be applied to inter predictionusing mask-based prediction shown in FIG. 8 .

FIG. 8 shows a concept of mask-based motion prediction and compensationproposed in the present disclosure. In the mask-based motion predictionand compensation proposed in the present disclosure, motion predictionand compensation are performed by applying a mask to reference blocks attwo or more different positions for one block.

In FIG. 8 , a method of obtaining a pixel from a reference block 820referred to by using a motion vector 830 for some pixels of a currentblock 801 in a picture 800 currently performing encoding and/ordecoding, and obtaining a pixel from a reference block 821 referred toby using a motion vector 831 for other pixels of the current block. Inthis case, in the process of obtaining some pixels of the current block,a method of referencing target pixels in the form of applying a weightto a pixel value at the same position of a corresponding reference blockmay be used.

FIG. 9 illustrates a concept of bidirectional prediction in a videoencoding and/or decoding method and apparatus according to an embodimentof the present disclosure.

In FIG. 9 showing bidirectional prediction in a video encoding anddecoding method and/or apparatus, in encoding and/or decoding a currentblock 910 existing in a current picture 900, a method of performingprediction using two or more reference blocks is referred to asbidirectional prediction.

In this case, the two or more reference blocks may be a block includedin reference pictures 901 and 902 that have been temporally encodedand/or decoded prior to a current picture, or a block included in aregion in which encoding and/or decoding is performed before a currentblock in a current picture.

In addition, one reference block may be a block included in a temporallyadjacent picture, and the other reference block may be a block includedin a region in which encoding and/or decoding is performed prior to acurrent block in a current picture.

As shown in FIG. 9 , in performing bidirectional prediction for acurrent block 910, a prediction block of the current block 910 may begenerated using different motion vectors mv0 (930), mv1 (931), orreference blocks 920 and 921 at positions derived using two or moremotion vectors.

In this case, the motion vector comprehensively refers to informationindicating a relative position of a reference block in a 2D coordinateplane with respect to a current block 910.

In addition, two or more motion vectors may be used.

FIG. 10 illustrates a concept of limited bidirectional prediction as anembodiment to which the present disclosure is applied.

In FIG. 10A showing a concept in which prediction for a L1 direction islimited in bidirectional prediction, in encoding and/or decoding acurrent block 910 existing in a current picture 900, a method offorcibly performing unidirectional prediction in one direction accordingto a specific condition is referred to as limited bidirectionalprediction.

In this case, according to an embodiment of the present disclosure, inthe forcibly performing unidirectional prediction for one direction, anadditional step of determining a predefined specific condition may beincluded.

The predefined specific condition may mean a size of a block and acondition for whether to use motion prediction in units of sub-blocks ofa corresponding block.

In addition, the predefined specific condition may indicate not only asize of a block and a condition for whether to use motion prediction inunits of sub-blocks of a corresponding block, but also a condition forwhether to use block division motion prediction that divides one blockinto a plurality of blocks such as affine motion prediction and diagonalblock division. A use of motion prediction in units of sub-blocks may beinterpreted in the same meaning as affine motion prediction.

In addition, the size of the block among the predefined specificconditions may mean a size of a block expressed as a sum of a width ofthe block and a height of the block, or a product of a width of theblock and a height of the block, and it may be predefined or transmittedto a decoder through higher-level syntax.

In this case, the size of the block, with respect to a sum of a width ofthe block and a height of the block, may be 12 derived from (8+4) or(4+8), or may be 16 derived from (8+8). In addition, the size of theblock may be 32 derived from (4×8) or (8×4), or may be 64 derived from(8×8) with respect to a product of a width of the block and a height ofthe block.

When using limited bidirectional prediction according to an embodimentof the present disclosure, even when a motion vector predictor (MVP) ofa current block 910 is bi-directional prediction, a step of forciblyadjusting motion vector information to perform unidirectional predictionmay be additionally included.

In an embodiment of the present disclosure shown in FIG. 10A, withrespect to a current block 910, an example of limiting reference to thereference picture 902 in a L1 direction and limiting a reference motionvector mv1 1002 and a reference block 1012 in the L1 direction isillustrated. In this case, it additionally includes a step of forciblyinitializing a reference motion vector mv1(1002) in a L1 direction to a(0,0) vector, initializing a reference picture index in a L1 direction,and changing a prediction direction of a current block 910 to mean L0during unidirectional prediction.

FIG. 10B shows a concept in which prediction for a L0 direction islimited in bidirectional prediction according to an embodiment of thepresent disclosure.

In FIG. 10B showing a concept in which prediction for a L0 direction islimited in bidirectional prediction, in encoding and/or decoding acurrent block 910 existing in a current picture 900, a method offorcibly performing unidirectional prediction for one direction under aspecific condition is called limited bidirectional prediction.

In this case, according to an embodiment of a present disclosure, inforcibly performing unidirectional prediction for one direction, anadditional step of determining a predetermined specific condition may beincluded.

The predefined specific condition may mean a size of a block and acondition for whether to use motion prediction in units of sub-blocks ofa corresponding block.

In addition, the predefined specific condition may indicate not only asize of a block a condition for whether to use motion prediction inunits of sub-blocks of a corresponding block, but also a condition forwhether to use block division motion prediction that divides one blockinto a plurality of blocks such as affine motion prediction and diagonalblock division. A use of motion prediction in units of sub-blocks may beinterpreted in the same meaning as affine motion prediction.

When using limited bidirectional prediction according to an embodimentof the present disclosure, even when a motion vector predictor (MVP) ofa current block 910 is bi-directional prediction, a step of forciblyadjusting motion vector information to perform unidirectional predictionmay be additionally included.

In the embodiment of the present disclosure shown in FIG. 10B, anexample of limiting a reference motion vector mv0 1003 and a referenceblock 1013 in a L0 direction by limiting reference to a referencepicture 901 in a L0 direction with respect a current block 910 is shown.In this case, it additionally includes a step of forcibly initializing areference motion vector mv0(1003) in a L0 direction to a (0,0) vector,initializing a reference picture index in a L0 direction, and changing aprediction direction of a current block 910 to mean L1 duringunidirectional prediction.

FIG. 11 is a flowchart of a process of decoding a predictiondirectionality according to an embodiment of the present disclosure.

FIG. 11 shows a flowchart of a process of decoding a predictiondirectionality of a current block, and it may be used in a motionprediction mode for decoding the prediction directionality of thecurrent block.

According to the embodiment shown in FIG. 11 , a process of decoding aprediction directionality may include at least one of determiningwhether a current slice type is a slice allowing bidirectionalprediction (S1110) or determining whether a current block is a blockallowing bidirectional prediction (S1120). In addition, it may befurther included at least one of a step of parsing a first bin of theprediction directionality (S1130) in the case that a current block is ablock that allows bidirectional prediction, or a step of parsing asecond bin of the prediction directionality (S1150) in the case that asize of a current block is not a size of a block allowing bidirectionalprediction or the first bin is 0.

In addition, it may be further included at least one of a step ofsetting a current prediction directionality as a bidirectionalprediction direction (S1170), a step of setting a current predictiondirectionality as a unidirectional prediction direction in a L1direction (S1180), or a step of setting a current predictiondirectionality as a unidirectional prediction direction in a L0direction (S1190).

In determining whether the current slice type is a slice type allowingbidirectional prediction (S1110), it is determined whether bidirectionalprediction is allowed by checking the current slice type.

In this case, a slice type that allows the bidirectional prediction maybe a B slice. In addition, in a prediction method using a pre-decodedregion of a current picture as a reference picture, the slice type maybe various.

A step of determining whether a current block is a block allowingbidirectional prediction (S1120) may mean an additional step ofdetermining a specific condition for a current block, wherein thespecific condition may mean a size of a block. In addition, it may beadditionally included one or more of conditions for whether to usesub-block unit motion prediction of a corresponding block, and whetherto use block division motion prediction that divides one block into aplurality of blocks, such as affine motion prediction and diagonal blockdivision.

In the embodiment shown in FIG. 11 , in a step of determining whether acurrent block is a block allowing bidirectional prediction (S1120), acase of determining bidirectional prediction using a size of the currentblock was used as an example, and it is determined whether the size ofthe current block is a size of a block that allows bidirectionalprediction, which is predefined or transmitted in higher-level syntax.

In this case, the size of the block that allows bidirectionalprediction, which is predefined or transmitted in higher-level syntax,may be expressed as a sum of a width of the block and a height of theblock, or a product of a width of the block and a height of the block.

In this case, the size of the block may be 12 derived from (8+4) or(4+8), or may be 16 derived from (8+8) with respect to a sum of a widthof the block and a height of the block. In addition, the size of theblock may be 32 derived from (4×8) or (8×4), or may be 64 derived from(8×8) with respect to a product of a width of the block and a height ofthe block.

When the size of the current block is a size of a block that allowsbidirectional prediction, a step of parsing a first bin for a predictiondirectionality (S1130) and subsequent steps are performed, and when thesize of the current block is not the size of a block that allowsbidirectional prediction, instead of performing the step of parsing thefirst bin for the prediction directionality (S1130) and a step ofdetermining a value of the first bin (S1140), a step of parsing a secondbin for a prediction directionality (S1150) is performed.

In the step of determining the value of the first bin for the predictiondirectionality (S1140), when the value of the first bin for theprediction directionality means 1, in a step of setting a predictiondirectionality of a current block as bidirectional prediction (S1170), acurrent prediction directionality is set to bidirectional direction.

When the size of the current block is not a size of a block that allowsbidirectional prediction, or when the value of the first bin for theprediction directionality is 0, a step of parsing a second bin for theprediction directionality (S1150) is performed. In addition, in the stepof determining the value of the second bin for the correspondingprediction directionality (S1160), when the value of the second bin forthe prediction directionality means 1, in a step of setting theprediction directionality of the current block to unidirectionalprediction in a L1 direction (S1180), the current directionalitydirection is set as unidirectional prediction in the L1 direction, andwhen the value of the second bin for the prediction directionality means0, in a step of setting the prediction directionality of the currentblock as unidirectional prediction in a L0 direction (S1190), thecurrent prediction directionality is set as unidirectional prediction inthe L0 direction.

Table 7 may be an example of a size of a block in which the size of thecurrent block allows bidirectional prediction. As shown in Table 7, whena block is divided using various block structures, when a sum of a widthand a height of a block is less than or equal to a specific value,bidirectional prediction is not used, and only when the sum of the widthand the height of the block is greater than the specific value, a limitmay be set so that bidirectional prediction can be used. Table 7 showsan example in which the specific value is defined as 12. The specificvalue may use a predefined value, or may be signaled in one or more of asequence parameter set (SPS), a picture parameter set (PPS), and a sliceheader, which are higher-level syntax.

TABLE 7 Name of inter_pred_idc ( cbWidth + ( cbWidth + cbHeight ) <=inter_pred_idc cbHeight ) > 12 12 0 PRED_L0 PRED_L0 1 PRED_L1 PRED_L1 2PRED_BI —

FIG. 12 is a flowchart for determining whether a predictiondirectionality is limited according to an embodiment of the presentdisclosure.

FIG. 12 is a flowchart illustrating a step of determining whether tolimit a prediction directionality of a current block, and whether tolimit the prediction directionality of the current block may be used notonly in a step of encoding and/or decoding the prediction directionalityof the current block, but also in a step of setting the predictiondirectionality of the current block.

The step of setting the prediction directionality of the current blockmay mean a case in which the prediction directionality of a predefinedmotion candidate is set as the prediction directionality of the currentblock without going through a step of encoding and/or decoding theprediction directionality of the current block when the current block isencoded and/or decoded in skip mode and/or merge mode.

According to the embodiment shown in FIG. 12 , a step of determiningwhether to limit a prediction directionality of a current block mayinclude at least one of a step of determining whether a current block isa block for performing motion prediction in units of sub-blocks (S1200)or a step of determining whether a size of a current block is a blocksize allowing bidirectional prediction (S1210). Additionally, when acurrent block is a limited bidirectional prediction block, a step ofadjusting a prediction directionality, a motion vector, and a referencepicture index of a current block may be included.

In determining whether the current block is a block for performingmotion prediction in units of sub-blocks (S1200), it is a step ofdetermining whether the current block is a mode in which motionprediction is performed in units of sub-blocks. The motion prediction inunits of sub-blocks refers to a prediction mode in which a current blockis divided into a plurality of sub-blocks and motion prediction isperformed with reference to different motion vectors for each sub-block.For example, if a current block performs motion prediction in units ofsub-blocks (e.g., affine mode), bidirectional prediction of a currentblock may not be limited, and if not (e.g., merge mode), bidirectionalprediction of a current block may be limited. Even when a current blockdoes not perform motion prediction in units of sub-blocks, it may bedetermined whether bidirectional prediction of a current block islimited by further considering a size of a current block, as describedlater.

In determining whether a size of a current block is a size of a blockthat allows bidirectional prediction (S1210), it is determined whether asize of a current block is a size of a block that allows bidirectionalprediction that is predefined or transmitted in higher-level syntax.

In this case, the size of the block that allows bidirectional predictionthat is predefined or transmitted in higher-level syntax may beexpressed as a sum of a width of the block and a height of the block, ora product of a width of the block and a height of the block.

In this case, the size of the block may be 12 derived from (8+4) or(4+8), or 16 derived from (8+8) with respect to a sum of a width of theblock and a height of the block. In addition, the size of the block maybe 32 derived from (4×8) or (8×4), or 64 derived from (8×8) with respectto a product of a width of the block and a height of the block.

For example, if a size of a current block is less than or equal to apredetermined threshold value, bidirectional prediction of a currentblock may be limited, otherwise, bidirectional prediction of a currentblock may not be limited. Herein, the threshold value may be differentaccording to a method of expressing a size of a current block, and maybe defined as 12, 16, 32, or 64 as described above.

When the current block is determined to be a limited bidirectionalprediction block, it may further include a step of adjusting aprediction direction, a motion vector, and a reference picture index ofthe current block. In an embodiment of the present disclosure, when acurrent block is determined to be a limited bidirectional predictionblock, a prediction directionality of the current block is set as aunidirectional prediction block in a L0 direction, a motion vector in aL1 direction is initialized with a (0,0) vector, and a reference pictureindex in the L1 direction is also initialized.

FIG. 13 is a flowchart for determining whether a predictiondirectionality is limited according to another embodiment of the presentdisclosure.

FIG. 13 is a flowchart illustrating a step of determining whether tolimit a prediction directionality of a current block, and whether tolimit a prediction directionality of a current block may be used notonly in a step of encoding and/or decoding the prediction directionalityof the current block, but also in a step of setting the predictiondirectionality of the current block. In addition, it may be used in astep of setting a prediction directionality of a sub-block in thecurrent block.

The step of setting the prediction directionality of the current blockmay mean a case in which the prediction directionality of a predefinedmotion candidate is set as the prediction directionality of the currentblock without going through a step of encoding and/or decoding theprediction directionality of the current block when the current block isencoded and/or decoded in skip mode and/or merge mode. In addition, itmay include a step of setting prediction directionalities for allsub-blocks in the current block.

According to the embodiment shown in FIG. 13 , a step of determiningwhether to limit a prediction directionality of a current block mayinclude at least one of a step of determining whether a size of thecurrent block is a size of a block allowing bidirectional prediction(S1310) or a step of determining whether the current block is a blockthat performs motion prediction in units of sub-blocks (S1320). Inaddition, at least one of steps of adjusting a predictiondirectionality, a motion vector, and a reference picture index of thecurrent block (S1330, S1340) or steps of adjusting predictiondirectionalities, motion vectors, and reference picture indexes of allsub-blocks in the current block (S1350, S1360) may be additionallyincluded.

In determining whether a size of a current block is a size of a blockthat allows bidirectional prediction (S1310), it is determined whether asize of a current block is a size of a block that allows bidirectionalprediction that is predefined or transmitted in higher-level syntax.

In this case, the size of the block that allows bidirectionalprediction, which is predefined or transmitted in higher-level syntax,may be expressed as a sum of a width of the block and a height of theblock, or a product of a width of the block and a height of the block.

In this case, the size of the block may be 12 derived from be (8+4) or(4+8), or 16 derived from (8+8), with respect to a sum of a width of theblock and a height of the block. In addition, the size of the block maybe 32 derived from (4×8) or (8×4), or 64 derived from (8×8) with respectto a product of a width of the block and a height of the block.

In determining whether a current block is a block for performing motionprediction in units of sub-blocks (S1320), it is a step of determiningwhether the current block is a mode in which motion prediction isperformed in units of sub-blocks. The motion prediction in units ofsub-blocks refers to a prediction mode in which a current block isdivided into a plurality of sub-blocks and motion prediction isperformed with reference to different motion vectors for each sub-block.

A step of adjusting a prediction directionality, a motion vector, and areference picture index of the current block (S1330 and S1340) may beincluded. In an embodiment of the present disclosure, when a currentblock is determined to be a limited bidirectional prediction block, aprediction directionality of the current block is set as aunidirectional prediction block in a L0 direction, a motion vector in aL1 direction is initialized with a (0,0) vector, and a reference pictureindex in the L1 direction is also initialized.

Steps (S1350 and S1360) of adjusting prediction directionalities, motionvectors, and reference picture indexes of all sub-blocks in the currentblock may be included. In an embodiment of the present disclosure, whena current block is determined to be a limited bidirectional predictionblock, prediction directionalities of all sub-blocks in the currentblock are set as a unidirectional prediction block in a L0 direction,motion vectors in a L1 direction are initialized with a (0,0) vector,and reference picture indices in the L1 direction are also initialized.

FIG. 14 illustrates a concept of dividing one picture using a unitdefined as multiple layers according to an embodiment of the presentdisclosure.

In decoding an image and a video, one image may be grouped into aplurality of image blocks and may be independently decoded. In thiscase, a predetermined fragment region may be defined by grouping theplurality of image blocks. Herein, the fragment region may mean at leastone of a sub-picture, a slice, a tile, a sub-tile (brick), or a CTU.

For example, one picture may be divided into one or more sub-pictures. Asub-picture may be composed of one or more slices, or may be composed ofone or more tiles. It may be limited so that one slice does not span aplurality of sub-pictures. Alternatively, it may be limited so that onetile does not span a plurality of sub-pictures.

For division into sub-pictures, one picture may be divided in verticaland horizontal directions at k-grid intervals. An index may be assignedto each grid. An index may be signaled for each grid. Grouping may beperformed for each grid with the same index. A group of grids with thesame index may be defined as a sub-picture. The k-grid refers to ksample units, and k may be 4, 8, 16, 32, 64 or more. The k-grid may bedetermined to be the same as a width and/or a height of a CTUpre-promised to an encoding/decoding apparatus.

The slice may be composed of one or more tiles, or may be composed ofone or more bricks. Slices may be replaced with the term tile group. Theslice may be specified based on a position of a top left tile (or brick)and a bottom right tile (or brick). In this case, the slice may beimplemented in a rectangular shape. A tile may be composed of one ormore of bricks. It will be described later using a term tile or tilegroup. However, a tile or a tile group may be understood as beingreplaced with a sub-picture, a slice, a tile, or a brick.

As shown in FIG. 14 , a single image may be reconstructed by dividing itin a rectangular shape such as Tile1_1 to Tile1_5. In this case, onetile may be divided into one or more tiles. This concept may be called asub-tile. Conversely, one or more tiles may be grouped together andcalled a tile group. Looking at this concept hierarchically, it may beexpressed as multi-level tile division. A tile refers to a group of oneor more coding tree units (CTU), where a bottom right boundary of thetile may not include a complete CTU. A Tile group and a sub-tile may ormay not have a rectangular shape. In a case of a tile group, an entirereconstructed image is determined according to an arrangement of tilesand a form of a bundle. In the case of a sub-tile, it may be representedas a bundle of a plurality of CTUs inside a tile, and may be rectangularor may have a shape of a slice according to a raster scanning order.

First, information for dividing one image into a plurality of tiles maybe delivered as a part of header information of a compressed bitstream.First, a video or image is reconstructed according to divisioninformation of tiles for one image group or one image. In expressing ashape of a tile, it may be expressed by the number of tiles and a widthand height of each tile. In this case, an order of indicating a tileshape information may be assumed to be a raster scanning order. In FIG.14 , when there are 5 tiles, a structure of a tile may be sequentiallyreconstructed at a receiving end by using information indicating that itis composed of five tiles first, and width and height informationstarting from the first tile.

In addition, each tile may be divided into sub-tiles, and at a receivingend, it is possible to reconstruct how each tile is composed ofsub-tiles by using information on how many sub-tiles each tile iscomposed of, and position and size information such as a width andheight of each sub-tile.

In addition, each tile may be grouped into several groups, and an imagemay be reconstructed in one bundle. The tile group may be represented bya method of expressing the number of tile groups and the tile number ororder of each tile group, and the structure of the tile group may bereconstructed at the receiving end.

A tile represents a unit capable of independently reconstructing animage. As a result, image reconstruction may be performed independentlyat the same time when bitstream information exists regardless of anorder of a plurality of tiles. According to an image group or an image,a mutually reconstructed image between tiles may or may not be referredto according to higher-level header information. In the case of a stillimage, each tile may be completely independently reconstructed or may bereconstructed through reference between tiles. As an example, when areconstructed image that is temporally identical and spatially differentis referred to, such as an IBC mode or intra prediction, reference maybe limited within a tile, and reference between tiles may be allowedaccording to higher-level information. Even in the case of motioncompensation, reference between tiles may not be possible according tohigher-level information, or reference between tiles may be allowed.Whether such a space or time reference is possible may be applied as thesame concept in a sub-tile. Entropy decoding may be performedindependently of each other, and in some cases, decoding may beperformed using some information of a previous sub-tile. It is possibleto disable or enable spatial and temporal prediction between thesesub-tiles. It may also be applied to the tile group mentioned above.Each tile may independently perform entropy decoding, but spatialtemporal prediction decoding may be performed only within a tile group.Whether to apply filtering to a boundary of a reconstructed imagebetween these tiles, between tile groups, or between sub-tiles isdetermined according to higher-level header information. Herein, thefiltering may refer to a filter such as a deblocking filter, a sampleadaptive offset (SAO), an adaptive loop filter (ALF), etc. included inan in-loop filter in an image encoding and decoding apparatus.

Herein, the sub-tile may be separately entropy-decoded like a tile.Alternatively, a first sub-tile may independently start decoding, and asubsequent consecutive sub-tile may perform entropy decoding using apart of decoding information of the previous tile. Alternatively, itmeans that entropy decoding may be performed using reconstructed imageinformation of a previous tile. This multi-level image division may beextended to multiple layers.

FIG. 15 illustrates a concept of dividing an image, an image group, or avideo into a plurality of tiles and processing them into three tilegroups according to an embodiment of the present disclosure.

In FIG. 15 , an image, an image group, and a video are divided into aplurality of tiles, when these are grouped into three tile groups, eachtile group is collected in a separate buffer, and only a reconstructedimage corresponding to a tile are shared, and an image corresponding tothe tile group may be restored.

In this case, each tile may be entropy-decoded separately or may bereconstructed in an interdependent manner. As previously described, itis possible to turn on/off spatiotemporal prediction through sharing ofreconstructed images and images between tiles, between tile groups, orbetween sub-tiles and the application of filtering to a boundary regionbetween them, and it may be signaled using a higher-level syntax.

In this embodiment, a tile group may be replaced by a sub-picture or aslice, and a tile and a sub-tile may be replaced by a slice and a brick,and the same may be applied to an embodiment described later.

FIG. 16 illustrates a concept in which an image, an image group, or animage is divided into a plurality of tiles, and only a reconstructedimage corresponding to each tile is shared and restored according to anembodiment of the present disclosure.

In FIG. 16 , an image, an image group, and a video are divided into aplurality of tiles, each tile is collected in a separate buffer, andonly a reconstructed image corresponding to a tile is shared, and animage corresponding to the tile may be reconstructed.

In this case, each tile may be entropy-decoded separately or may bereconstructed in an interdependent manner. As previously described, itis possible to turn on/off spatiotemporal prediction through sharing ofreconstructed images and images between tiles or between sub-tiles andthe application of filtering to a boundary region between them, and itmay be signaled using a higher-level syntax.

FIG. 17 illustrates an example in which an image, an image group, or animage are divided into a plurality of tiles, and each tile is dividedinto sub-tiles according to an embodiment of the present disclosure.

FIG. 17 shows an example in which one image, an image group, and a videoare divided into a plurality of tiles, and each tile is divided intosub-tiles again. In this case, sub-tiles are collected in a separatebuffer, and only a reconstructed image corresponding to a tile isshared, and an image corresponding to the tile may be reconstructed.

In this case, each sub-tile may be entropy-decoded separately or may bereconstructed in an interdependent manner. Interdependent manner refersto referencing update information of a context table for entropydecoding. As previously described, it is possible to turn on/offspatiotemporal prediction through sharing of reconstructed images andimages between tiles or between sub-tiles and the application offiltering to a boundary region between them, and it may be signaledusing a higher-level syntax.

HRD (Hypothetical Reference Decoder) mutually determines a size of abuffer and delay information on a decoder side in order to smoothlyrestore and display a video in real time by receiving a compressedbitstream between a decoder and an encoder at a limited rate.Accordingly, in order to receive and restore data at a limited rate, andto output it in real time, on the decoder side, according to informationsuch as a size of a buffer defined in a bitstream, a delay time, and atransmission rate, it is also necessary to set the decoder to performthe restoration. A size of a buffer, a delay time, a transmission rate,or the amount of filling an initial buffer may be defined, and with someof these information, other information may be obtained in aproportional manner.

It has been described that the aforementioned multi-level tiles ormulti-layer tiles may be independently or dependently encoded ordecoded. HRD may be defined in multiple levels with information on sucha multi-level tile or a multi- layer tile. HRD for an image, an imagegroup, or an entire video may be defined. In addition, HRD informationmay be defined for each tile, and accordingly, each tile mayindependently change the amount of buffer and delay. Alternatively, asize of a buffer, a delay, the amount of an initial buffering, and atransmission rate may be considered. By extending this concept, such HRDmay be adjusted for each tile group. In addition, according to thisconcept, HRD may be separately set for a sub-tile, and accordingly, adecoder may reconstruct an image or video according to information suchas a size of a buffer and a delay according to a separate HRD. Inaddition, it may be possible to infer these multi-level HRD rulesbetween levels. That is, when HRDs are defined for each sub-tile, theHRD for a tile may be inferred as an average, maximum, minimum, and sumof HDR information for the sub-tiles. For example, a size of a bufferfor a corresponding tile may be set with a size obtained by summingbuffer sizes of sub-tiles. Alternatively, in the case of an initialdelay for a tile, the maximum value of initial delays for sub-tiles maybe used. This hierarchical HRD may be transmitted for an image, an imagegroup, or an entire video. In particular, in the case of an image group,it may be defined in a random access point image. In the case of two-wayimage communication, HRD may be defined according to terminalperformance by exchanging the terminal performance in advance betweendevices or between a terminal and a server. For the convenience oftransmitting the hierarchical HRD information, HRD may be definedaccording to the number of tiles, the number of tile groups, and thenumber of sub-tiles according to tile structure information.Alternatively, whether to define HRD according to a tile, and a size ofa buffer and a delay for each HRD may be defined, or shared informationmay be used.

In the case of a special video such as a 360-degree video, for one imageor video, only tiles and tile groups of only a portion of the area maybe reconstructed. In this case, according to tile information and crossreference information, a bitstream buffer of a decoder and areconstructed image may be used separately. In addition, in a video, atile or tile group to be selectively reconstructed may be changedaccording to time, and accordingly, HRD must be selectively configuredto receive and reconstruct an image.

Video compression and reconstruction is divided into anintra-encoded/decoded image and an inter-encoded/decoded image. In thiscase, an intra prediction and reconstruction technology is included inan inter-encoded/decoded image, but an inter prediction andreconstruction technique is not applied to an intra-encoded/decodedimage. In addition, an intra-encoding/decoding slice and aninter-encoding/decoding slice are defined, an intra-encoding/decodingslice uses only intra-block encoding/decoding technology, and aninter-encoding/decoding slice uses both intra-block and inter-blockencoding/decoding technologies. In addition, an intra tile may use onlyan intra-block encoding/decoding technology, and an inter tile may useboth an intra-block encoding/decoding technology and an inter-blockencoding/decoding technology. In addition, in an inter-tile, intraprediction reconstruction without using information reconstructed by aneighboring inter-block coding/decoding technology, such as CIP(Constraint intra prediction), may be performed. A shape of the intratile is defined by several patterns, so that after several image frameshave passed, a subsequent image may be reconstructed regardless of aprevious error. In addition, a decoder side may remove an imagereconstructed before a pattern period from DPB at the time when theimage is completely reconstructed according to a pattern of an intratile or CIP.

Various embodiments of the present disclosure are not listed as listingall possible combinations, but are intended to describe representativeaspects of the present disclosure, and matters described in the variousembodiments may be applied independently or may be applied incombination of two or more.

In addition, various embodiments of the present disclosure may beimplemented by hardware, firmware, software, or a combination thereof.In the case of implementation by hardware, it can be implemented by oneor more Application Specific Integrated Circuits (ASICs), Digital SignalProcessors (DSPs), Digital Signal Processing Devices (DSPDs),Programmable Logic Devices (PLDs), Field Programmable Gate Arrays(FPGAs), general processors, controllers, microcontroller,microprocessor, etc.

The scope of the present disclosure includes software ormachine-executable instructions (e.g., operating system, application,firmware, program, etc.) that allow an operation according to a methodof various embodiments to be executed on a device or a computer, and anon-transitory computer-readable medium in which the software orinstructions are stored and executed on a device or a computer.

INDUSTRIAL AVAILABILITY

The present disclosure may be used to encode/decode an image.

The invention claimed is:
 1. A method of decoding an image, the methodcomprising: deriving, based on an inter mode pre-defined in an imagedecoding apparatus, motion information for bi-directional prediction ofa current block in the image; and performing inter prediction on thecurrent block based on the motion information, wherein, according to thepre-defined inter mode, the motion information for the bi-directionalprediction is adjusted to motion information for uni-directionalprediction, wherein, in response to a case where the pre-defined intermode is a merge mode, the motion information for the bi-directionalprediction is adjusted to the motion information for the uni-directionalprediction, and wherein, in response to a case where the pre-definedinter mode is an affine mode, the motion information for thebi-directional prediction is not adjusted to the motion information forthe uni-directional prediction.
 2. The method of claim 1, wherein, inresponse to the case where the pre-defined inter mode is the merge mode,the motion information for the bi-directional prediction is adjusted tothe motion information for the uni-directional prediction inconsideration of a size of the current block.
 3. The method of claim 2,wherein, in response to the case where the pre-defined inter mode is themerge mode, deriving the motion information comprises: constructing amerge candidate list of the current block; and deriving the motioninformation of the current block from the merge candidate list, whereinthe merge candidate list includes at least one of a spatial mergecandidate, a temporal merge candidate, or a combined merge candidate. 4.The method of claim 1, wherein, in response to the case where thepre-defined inter mode is the affine mode, deriving the motioninformation comprises: generating a candidate list consisting of affinecandidates of the current block; deriving a control point vector of thecurrent block based on the candidate list and a candidate index; andderiving a motion vector of the current block based on the control pointvector of the current block.
 5. The method of claim 4, wherein theaffine candidates include at least one of a spatial candidate, atemporal candidate, or a configured candidate.
 6. The method of claim 4,wherein, in consideration of a size of a predetermined reference region,the motion vector of the current block is derived, and wherein thereference region includes a reference block specified by motion vectorsof n sub-blocks and a pixel used to interpolate a fractional pel of thereference block.
 7. The method of claim 6, wherein, in response to acase where the size of the reference region is less than or equal to athreshold value, the motion vector of the current block is derived foreach sub-block, and wherein, in response to a case where the size of thereference region is greater than the threshold value, one motion vectoris derived for the current block.
 8. The method of claim 7, wherein themotion vector of the current block is derived based on an affine motionparameter, and wherein the affine motion parameter is calculated basedon the control point vector of the current block.
 9. The method of claim8, wherein the current block belongs to a predetermined sub-picture, andwherein the sub-picture is defined as a group of grids having the sameindex among a plurality of grids constituting one picture.
 10. A methodof encoding an image, the method comprising: determining, based on aninter mode pre-defined in an image encoding apparatus, motioninformation for bi-directional prediction of a current block in theimage; and performing inter prediction on the current block in responseto the motion information, wherein, according to the pre-defined intermode, the motion information for the bi-directional prediction isadjusted to motion information for uni-directional prediction, wherein,in response to a case where the pre-defined inter mode is a merge mode,the motion information for the bi-directional prediction is adjusted tothe motion information for the uni-directional prediction, and wherein,in response to a case where the pre-defined inter mode is an affinemode, the motion information for the bi-directional prediction is notadjusted to the motion information for the uni-directional prediction.11. A non-transitory computer-readable medium for storing a bitstreamgenerated by an image encoding method, the image encoding methodcomprising: determining, based on an inter mode pre-defined, motioninformation for bi-directional prediction of a current block; andperforming inter prediction on the current block in response to themotion information, wherein, according to the pre-defined inter mode,the motion information for the bi-directional prediction is adjusted tomotion information for uni-directional prediction, wherein, in responseto a case where the pre-defined inter mode is a merge mode, the motioninformation for the bi-directional prediction is adjusted to the motioninformation for the uni-directional prediction, and wherein, in responseto a case where the pre-defined inter mode is an affine mode, the motioninformation for the bi-directional prediction is not adjusted to themotion information for the uni-directional prediction.